If an event takes ages to persist it blocks other events from being notified

[matrixbot]

This issue has been migrated from #16633.

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This is because we assign a stream ordering fairly early on, and we block notifying /sync streams / replication / etc about later events until the slow event has finished persisting.

One mitigation could be to assign a stream ordering as late as possible inside the transaction, to minimise the number of queries that could block future events.