Keep track when we try to backfill an event

[MadLittleMods]

Mentioned in internal doc about speeding up /messages. Related to #13356

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Keep track when we try to backfill an event. In other words, when an event fails to backfill, we can record it in the database.

This is a building block so we can later:

1. Only try to backfill from an event if we haven't tried recently -> Only try to backfill event if we haven't tried before recently #13622
2. When we decide to backfill that event again, process it in the background so it doesn't block and make /messages slow when we know it will probably fail again -> Process previously failed backfill events in the background #13623