Cross-signing [4/4] -- federation edition

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

make notification of signatures work with workers

* update version of black and also fix the mypy config being overridden

Add StateGroupStorage interface

* Offer the homeserver instance to the spam checker

* Newsfile

* Linting

* Expose a Spam Checker API instead of passing the homeserver object

* Alter changelog

* s/hs/api

Fix /purge_room admin API

Fix docstring

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

It fails trying to clean the `topic` table which was recently removed.

Replace every instance of `logger.warn` with `logger.warning` as the former is deprecated.

this is part of _retry_on_integrity_error, so should only be on _persist_events_and_state_updates

Move persist_events out from main data store.

Make ObservableDeferred.observe() always return deferred.

This makes it easier to use in an async/await world.

Also fixes a bug where cache descriptors would occaisonally return a raw
value rather than a deferred.