We keep trying to fetch events whose keys have expired or have been invalidated

[matrixbot]

This issue has been migrated from #13676.

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Sometimes we try to fetch an event over federation (notably, if it forms part of the state or auth chain), and we try to validate its signature,

However, if we are unable to perform that validation, then we discard the event; but we might do the same again almost immediately. Possibly we need some sort of backoff.