* Limit AS transactions to 100 events

* Update changelog.d/8606.feature

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Add tests

* Update synapse/appservice/scheduler.py

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

Optionally sends typing, presence, and read receipt information to appservices.

Get rid of the labyrinthine `recoverer_fn` code, and clean up the startup code
(it seemed to be previously inexplicably split between
`ApplicationServiceScheduler.start` and `_Recoverer.start`).

Add some docstrings too.

... which were making other, innocent, tests, fail.

Plus remove a spurious unittest.DEBUG which was making the output noisy.

Replace EventGrouper for ServiceQueuer to move to push-based txns. Fix tests and add stub tests for ServiceQueuer.

With tests to assert behaviour. Not hooked up yet. Stub datastore methods
not implemented yet.