A sync call may not have all events that happened since the last time
the client synced. In such a case the room is marked as limited and
events need to be fetched separately.
When such a sync call happens our event index will have a gap. To
close the gap checkpoints are added to start crawling our room again.
Unnecessary full re-crawls are prevented by checking if our current
/room/roomId/messages request contains only events that were already
present in our event index.
4acec19d40