Add concept of cache contexts

Make get_new_events_for_appservice use indices

3rd party entity lookup

Move validation logic for AS 3PE query response into ApplicationServiceApi class, to keep the handler logic neater

Avoid so much copypasta between 3PU and 3PL query by unifying around a ThirdPartyEntityKind enumeration

Add appservice worker

Make AppserviceHandler stream events from database

This is for two reasons:

1. Suppresses duplicates correctly, as the notifier doesn't do any
   duplicate suppression.
2. Makes it easier to connect the AppserviceHandler to the replication
   stream.

Add a media repository worker

Make notify_interested_services faster

Fix push_display_name_rename schema update