This ended up ballooning much larger than intended :/
Part of the reason for the large amount of changes was the elimination
of the _ServiceQueuer.enqueue_{event,ephemeral} methods and replacing
them with ApplicationServiceScheduler.enqueue_for_appservice.
That meant we had to change all of the tests that mock'd
enqueue_{event,ephemeral} to instead look lower-level, at the calls to
_TransactionController.send.
Other than that, there was just the normal amount of adding checks that
to_device_messages as an argument to enqueue_for_appservice is filled
out correctly.
The change to scheduler means that we don't have to keep adding another
[] every time we check a call to _TransactionController.send.
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