Profile /messages and make it even faster v2

[kittykat]

Message history can be very slow to load. Users expect to be able to rapidly load scrollback within a room, infinite scrolling without having to wait for the server to load more messages.

This is the follow on task from the Q4 one: #14284

These tasks need to be completed for this issue to be closed:

• Investigate why /messages is slow #13356
• Process previously failed backfill events in the background #13623
• Make the backfill linearizer lock smarter to not block as often #13619
• Generalize tracking whenever we fail to fetch or process a federated event #13700
• /state_ids is slow to respond #13620
• have_seen_events is eating up time while processing state and auth events while backfilling #13625
• Add limit to get_current_hosts_in_room #13659
• We keep trying to fetch events whose keys have expired or have been invalidated #13676

[erikjohnston]

Closing this as we're not actively working on this anymore