* Make historical messages available to federated servers

Part of MSC2716: https://github.com/matrix-org/matrix-doc/pull/2716

Follow-up to https://github.com/matrix-org/synapse/pull/9247

* Debug message not available on federation

* Add base starting insertion point when no chunk ID is provided

* Fix messages from multiple senders in historical chunk

Follow-up to https://github.com/matrix-org/synapse/pull/9247

Part of MSC2716: https://github.com/matrix-org/matrix-doc/pull/2716

---

Previously, Synapse would throw a 403,
`Cannot force another user to join.`,
because we were trying to use `?user_id` from a single virtual user
which did not match with messages from other users in the chunk.

* Remove debug lines

* Messing with selecting insertion event extremeties

* Move db schema change to new version

* Add more better comments

* Make a fake requester with just what we need

See https://github.com/matrix-org/synapse/pull/10276#discussion_r660999080

* Store insertion events in table

* Make base insertion event float off on its own

See https://github.com/matrix-org/synapse/pull/10250#issuecomment-875711889

Conflicts:
	synapse/rest/client/v1/room.py

* Validate that the app service can actually control the given user

See https://github.com/matrix-org/synapse/pull/10276#issuecomment-876316455

Conflicts:
	synapse/rest/client/v1/room.py

* Add some better comments on what we're trying to check for

* Continue debugging

* Share validation logic

* Add inserted historical messages to /backfill response

* Remove debug sql queries

* Some marker event implemntation trials

* Clean up PR

* Rename insertion_event_id to just event_id

* Add some better sql comments

* More accurate description

* Add changelog

* Make it clear what MSC the change is part of

* Add more detail on which insertion event came through

* Address review and improve sql queries

* Only use event_id as unique constraint

* Fix test case where insertion event is already in the normal DAG

* Remove debug changes

* Add support for MSC2716 marker events

* Process markers when we receive it over federation

* WIP: make hs2 backfill historical messages after marker event

* hs2 to better ask for insertion event extremity

But running into the `sqlite3.IntegrityError: NOT NULL constraint failed: event_to_state_groups.state_group`
error

* Add insertion_event_extremities table

* Switch to chunk events so we can auth via power_levels

Previously, we were using `content.chunk_id` to connect one
chunk to another. But these events can be from any `sender`
and we can't tell who should be able to send historical events.
We know we only want the application service to do it but these
events have the sender of a real historical message, not the
application service user ID as the sender. Other federated homeservers
also have no indicator which senders are an application service on
the originating homeserver.

So we want to auth all of the MSC2716 events via power_levels
and have them be sent by the application service with proper
PL levels in the room.

* Switch to chunk events for federation

* Add unstable room version to support new historical PL

* Messy: Fix undefined state_group for federated historical events

```
2021-07-13 02:27:57,810 - synapse.handlers.federation - 1248 - ERROR - GET-4 - Failed to backfill from hs1 because NOT NULL constraint failed: event_to_state_groups.state_group
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/synapse/handlers/federation.py", line 1216, in try_backfill
    await self.backfill(
  File "/usr/local/lib/python3.8/site-packages/synapse/handlers/federation.py", line 1035, in backfill
    await self._auth_and_persist_event(dest, event, context, backfilled=True)
  File "/usr/local/lib/python3.8/site-packages/synapse/handlers/federation.py", line 2222, in _auth_and_persist_event
    await self._run_push_actions_and_persist_event(event, context, backfilled)
  File "/usr/local/lib/python3.8/site-packages/synapse/handlers/federation.py", line 2244, in _run_push_actions_and_persist_event
    await self.persist_events_and_notify(
  File "/usr/local/lib/python3.8/site-packages/synapse/handlers/federation.py", line 3290, in persist_events_and_notify
    events, max_stream_token = await self.storage.persistence.persist_events(
  File "/usr/local/lib/python3.8/site-packages/synapse/logging/opentracing.py", line 774, in _trace_inner
    return await func(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/persist_events.py", line 320, in persist_events
    ret_vals = await yieldable_gather_results(enqueue, partitioned.items())
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/persist_events.py", line 237, in handle_queue_loop
    ret = await self._per_item_callback(
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/persist_events.py", line 577, in _persist_event_batch
    await self.persist_events_store._persist_events_and_state_updates(
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/databases/main/events.py", line 176, in _persist_events_and_state_updates
    await self.db_pool.runInteraction(
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/database.py", line 681, in runInteraction
    result = await self.runWithConnection(
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/database.py", line 770, in runWithConnection
    return await make_deferred_yieldable(
  File "/usr/local/lib/python3.8/site-packages/twisted/python/threadpool.py", line 238, in inContext
    result = inContext.theWork()  # type: ignore[attr-defined]
  File "/usr/local/lib/python3.8/site-packages/twisted/python/threadpool.py", line 254, in <lambda>
    inContext.theWork = lambda: context.call(  # type: ignore[attr-defined]
  File "/usr/local/lib/python3.8/site-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/local/lib/python3.8/site-packages/twisted/python/context.py", line 83, in callWithContext
    return func(*args, **kw)
  File "/usr/local/lib/python3.8/site-packages/twisted/enterprise/adbapi.py", line 293, in _runWithConnection
    compat.reraise(excValue, excTraceback)
  File "/usr/local/lib/python3.8/site-packages/twisted/python/deprecate.py", line 298, in deprecatedFunction
    return function(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/twisted/python/compat.py", line 403, in reraise
    raise exception.with_traceback(traceback)
  File "/usr/local/lib/python3.8/site-packages/twisted/enterprise/adbapi.py", line 284, in _runWithConnection
    result = func(conn, *args, **kw)
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/database.py", line 765, in inner_func
    return func(db_conn, *args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/database.py", line 549, in new_transaction
    r = func(cursor, *args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/synapse/logging/utils.py", line 69, in wrapped
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/databases/main/events.py", line 385, in _persist_events_txn
    self._store_event_state_mappings_txn(txn, events_and_contexts)
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/databases/main/events.py", line 2065, in _store_event_state_mappings_txn
    self.db_pool.simple_insert_many_txn(
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/database.py", line 923, in simple_insert_many_txn
    txn.execute_batch(sql, vals)
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/database.py", line 280, in execute_batch
    self.executemany(sql, args)
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/database.py", line 300, in executemany
    self._do_execute(self.txn.executemany, sql, *args)
  File "/usr/local/lib/python3.8/site-packages/synapse/storage/database.py", line 330, in _do_execute
    return func(sql, *args)
sqlite3.IntegrityError: NOT NULL constraint failed: event_to_state_groups.state_group
```

* Revert "Messy: Fix undefined state_group for federated historical events"

This reverts commit 187ab28611546321e02770944c86f30ee2bc742a.

* Fix federated events being rejected for no state_groups

Add fix from https://github.com/matrix-org/synapse/pull/10439
until it merges.

* Adapting to experimental room version

* Some log cleanup

* Add better comments around extremity fetching code and why

* Rename to be more accurate to what the function returns

* Add changelog

* Ignore rejected events

* Use simplified upsert

* Add Erik's explanation of extra event checks

See https://github.com/matrix-org/synapse/pull/10498#discussion_r680880332

* Clarify that the depth is not directly correlated to the backwards extremity that we return

See https://github.com/matrix-org/synapse/pull/10498#discussion_r681725404

* lock only matters for sqlite

See https://github.com/matrix-org/synapse/pull/10498#discussion_r681728061

* Move new SQL changes to its own delta file

* Clean up upsert docstring

* Bump database schema version (62)