Backfill rooms on refresh with no messages

[CarlAmbroselli]

When Facebook throws RateLimiting errors, it's common for the bridge to create the room, but no messages in it. If users then use refresh in the bridge room or restarts the bridge, they expect that the bridge continues backfilling messages where it had left of. Fixes #182.

[CarlAmbroselli]

@tulir would you mind reviewing? Have that version running since a week now and had no issues.

[tulir]

Rate limits should be handled better in the new backfill system