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

Remove SnapshotCache in favour of ResponseCache

Make the PusherSlaveStore inherit from the slave RoomStore

So that it has access to the get_retention_policy_for_room function which is required by filter_events_for_client.