Update the send_leave path to be an event_id

Stop populating events.content

* attempt at deduplicating lazy-loaded members

as per the proposal; we can deduplicate redundant lazy-loaded members
which are sent in the same sync sequence. we do this heuristically
rather than requiring the client to somehow tell us which members it
has chosen to cache, by instead caching the last N members sent to
a client, and not sending them again.  For now we hardcode N to 100.
Each cache for a given (user,device) tuple is in turn cached for up to
X minutes (to avoid the caches building up).  For now we hardcode X to 30.

* add include_redundant_members filter option & make it work

* remove stale todo

* add tests for _get_some_state_from_cache

* incorporate review

Remove some redundant joins on event_edges.room_id

It's still not used, however the parameter is an event ID not a transaction ID.

Fix some looping_call calls which were broken in #3604

There's no real point in ever making the column non-nullable, and doing so
breaks the sytests.

This field is no longer read from, so we should stop populating it. Once we're
happy that this doesn't break everything, and a rollback is unlikely, we can
think about dropping the column.

We've long passed the point where it's possible to have the same event_id in
different tables, so these join conditions are redundant: we can just join on
event_id.

event_edges is of non-trivial size, and the room_id column is wasteful, so
let's stop reading from it. In future, we can stop writing to it, and then drop
it.

It turns out that looping_call does check the deferred returned by its
callback, and (at least in the case of client_ips), we were relying on this,
and I broke it in #3604.

Update run_as_background_process to return the deferred, and make sure we
return it to clock.looping_call.

Implement the lazy_load_members room state filter parameter

Fix a minor documentation typo in on_make_leave

 Correctly handle outliers during persist events

Fix another logcontext leak in _persist_events

Fix occasional 'tuple index out of range' error

Fix updating of cached remote profiles

Wrap a number of things that run in the background

This fixes a bug in _delete_existing_rows_txn which was introduced in #3435
(though it's been on matrix-org-hotfixes for *years*). This code is only called
when there is some sort of conflict the first time we try to persist an event,
so it only happens rarely. Still, the exceptions are annoying.

We need to run the errback in the sentinel context to avoid losing our own
context.

Also: add logging to runInteraction to help identify where "Starting db
connection from sentinel context" warnings are coming from

_update_remote_profile_cache was missing its `defer.inlineCallbacks`, so when
it was called, would just return a generator object, without actually running
any of the method body.

on_notifier_poke no longer runs synchonously, so we have to do a different hack
to make sure that the replication data has been sent. Let's actually listen for
its arrival.