Modify sync transaction behaviour (#2758)

This now uses a transaction per stream, so that errors in one stream
don't propagate to another, and we therefore no longer need to do hacks
to reopen a new transaction after aborting a failed one.

syncapi/streams/stream_receipt.go

@@ -52,7 +52,6 @@ func (p *ReceiptStreamProvider) IncrementalSync(
 	lastPos, receipts, err := snapshot.RoomReceiptsAfter(ctx, joinedRooms, from)
 	if err != nil {
 		req.Log.WithError(err).Error("p.DB.RoomReceiptsAfter failed")
-		_ = snapshot.Reset()
 		return from
 	}