Part of: MSC4354
Part of: #19641
Follows: #20232 (which applies already-persisted redactions to Sticky
Events being persisted)
In some cases we batch events together during persistence.
If one of the events in the batch is a redaction of another event, we
should apply it, which provides the same benefit as #20232 (namely: we
avoid potentially dodgy event content being persisted & the event
naturally loses its stickiness
so is not written to `sticky_events`)
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Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>