AS scheduler may drop events during a restart

This issue has been migrated from [#11637](https://github.com/matrix-org/synapse/issues/11637).

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### Description

The AS scheduler keeps track of events to send to each AS in memory as they come in, only pushing these into the database (as an AS transaction) once any in-flight requests have been completed:

https://github.com/matrix-org/synapse/blob/c500bf37d660b08efb48501b7690dc4448b39eca/synapse/appservice/scheduler.py#L149-L173

### Steps to reproduce

This means AS events could be lost during the following series of events:

+ events come in, AS request begins
+ more events come in while AS is processing, these only exist in memory
+ a restart/crash/etc of the synapse or AS pusher process - the above AS events are lost forever

### Possible solution

Would it be possible to setup some kind of exit handling (`atexit` from stdlib?) that dumps any in-memory events into a new txn in the database before the process exits, this would prevent any loss of AS events.