Implement MSC2326 (Label based filtering)

[babolivier]

Implement matrix-org/matrix-spec-proposals#2326 in Synapse

PRs:

• Implement MSC2326 (label based filtering) #6301 (basic support)
• MSC2326: Add background update to take previous events into account #6310 (background job to support previous events)
• Filter state, events_before and events_after in /context requests #6329 (support on /context, also on /search and some test refactoring)
• Fix the SQL SELECT query in _paginate_room_events_txn #6340 (fix SQL used to query events when paginating)
• Support edits (was Make label-based filtering support edits #6374)

[richvdh]

PR is #6301

[babolivier]

#6374 is the wrong approach for handling edits, in that we can't ensure that updates arrive in the right order, or that updates even arrive after the original event. A correct implementation would be calculating the list of labels when processing edits when the client requests events.