* Initial cut for backfilling
The syncserver now asks the roomserver via QueryBackfill (which already
existed to *handle* backfill requests) which then makes federation requests
via gomatrixserverlib.RequestBackfill.
Currently, tests fail on subsequent /messages requests because we don't know
which servers are in the room, because we are unable to get state snapshots
from a backfilled event because that code doesn't exist yet.
* WIP backfill, doesn't work
* Make initial backfill pass checks
* Persist backfilled events with state snapshots
* Remove debug lines
* Linting
* Review comments
