When we request the state/auth_events to populate a backwards extremity (on
backfill or in the case of missing events in a transaction push), we should
check that the returned events are in the right room rather than blindly using
them in the room state or auth chain.

Given that _get_events_from_store_or_dest takes a room_id, it seems clear that
it should be sanity-checking the room_id of the requested events, so let's do
it there.

Move database config from apps into HomeServer object

Make it return the state *after* the requested event, rather than the one
before it. This is a bit easier and requires fewer calls to
get_events_from_store_or_dest.

Port some of FederationHandler to async/await

Port handlers.account_validity to async/await.

also fix user_joined_room to consistently return deferreds

... and _get_events_from_store_or_dest

and associated functions:
 * on_receive_pdu
 * handle_queued_pdus
 * get_missing_events_for_pdu

PaginationHandler.get_messages is only called by RoomMessageListRestServlet,
which is async.

Chase the code path down from there:
 - FederationHandler.maybe_backfill (and nested try_backfill)
 - FederationHandler.backfill

This just makes some of the logging easier to follow when things start going
wrong.

Synapse 1.7.0rc2 (2019-12-11)
=============================

Bugfixes
--------

- Fix incorrect error message for invalid requests when setting user's avatar URL. ([\#6497](https://github.com/matrix-org/synapse/issues/6497))
- Fix support for SQLite 3.7. ([\#6499](https://github.com/matrix-org/synapse/issues/6499))
- Fix regression where sending email push would not work when using a pusher worker. ([\#6507](https://github.com/matrix-org/synapse/issues/6507), [\#6509](https://github.com/matrix-org/synapse/issues/6509))

Implement v2 APIs for send_join and send_leave

Silence mypy errors for files outside those specified

This is a non-functional refactor as a precursor to some other work.

Phone home stats DB reporting should not assume a single DB.

Stop the `update_client_ips` background job from recreating deleted devices.

There was a bunch of unnecessary conversion back and forth between dict and
list going on here. We can simplify a bunch of the code.

Fix `make_deferred_yieldable` to work with coroutines

Fix support for SQLite 3.7.

Give the server config to the RoomWorkerStore