Add complete documentation of the message retention policies support

The mount in the form of ./matrix-config:/etc overwrites the contents of the container /etc folder. Since all valid ca certificates are stored in /etc, the synapse.push.httppusher, for example, cannot validate the certificate from matrix.org.

e.g. if remote returns a 404 then that shouldn't be treated as an error
but should be proxied through.

Fix a typo in the purge jobs configuration example

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

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

Have a purge job running every 5min is probably not something we want to advise admins to do as a sort-of default.

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