_auth_and_persist_outliers could do with being optimised

[matrixbot]

This issue has been migrated from #11013.

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As part of my work on #9595, I've ended up making some of the code which persists events received over federation a bit more conservative. (Specifically, this is the code for handling batches of outliers, in FederationEventHandler._auth_and_persist_outliers).

matrix-org/synapse#10896 (comment) raised the concern that this could result in a lot of round-trips between the federation inbound worker and the event persister.

It should be possible to optimise all this, so that we do all the event auth first, and then persist everything in one big chunk.