Implement backfill in the roomserver (#983)

* 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

federationapi/routing/backfill.go

@@ -69,6 +69,7 @@ func Backfill(
 
 	// Populate the request.
 	req := api.QueryBackfillRequest{
+		RoomID:            roomID,
 		EarliestEventsIDs: eIDs,
 		ServerName:        request.Origin(),
 	}