Process previously failed backfill events in the background because they are bound to fail again and we don't need to waste time holding up the request for something that is bound to fail again. Fix https://github.com/matrix-org/synapse/issues/13623 Follow-up to https://github.com/matrix-org/synapse/issues/13621 and https://github.com/matrix-org/synapse/issues/13622 Part of making `/messages` faster: https://github.com/matrix-org/synapse/issues/13356
