Fixes #6552 

This was ill-advised. We can't modify verify_keys here, because the response
object has already been signed by the requested key.

Furthermore, it's somewhat unnecessary because existing versions of Synapse
(which get upset that the notary key isn't present in verify_keys) will fall
back to a direct fetch via `/key/v2/server`.

Also: more tests for fetching keys via perspectives: it would be nice if we actually tested when our fetcher can't talk to our notary impl.

Fix conditions failing if min_depth = 0

* Add a background update to clear tombstoned rooms from the directory

* use the ABC metaclass

so that bg update routines can be async

Fix exceptions in the synchrotron worker log when events are rejected.

This could result in Synapse not fetching prev_events for new events in the room if it has missed some events.

Remove a bunch of unused code from event creation

Make it clearer how they behave in the face of rejected and/or missing events.

Fixes #4026

replace get_prev_events_and_hashes_for_room with get_prev_events_for_room in create_new_client_event

... to make way for a new method which just returns the event ids

Lift the restriction that *all* the keys used for signing v2 key responses be
present in verify_keys.

Fixes #6596.

* Kill off redundant SynapseRequestFactory

We already get the Site via the Channel, so there's no need for a dedicated
RequestFactory: we can just use the right constructor.

* Workaround for error when fetching notary's own key

As a notary server, when we return our own keys, include all of our signing
keys in verify_keys.

This is a workaround for #6596.

Modify a copy of an upgraded room's PL before sending to the new room

This has caused some confusion for people who didn't notice it going away.

This looks like it got half-killed back in #888.

Fixes #6567.

We already get the Site via the Channel, so there's no need for a dedicated
RequestFactory: we can just use the right constructor.

put the 2nd part of the "source installation"-section into a new section, because it also applies to Debian packages