Implement the /threads endpoint from MSC3856.
This is currently unstable and behind an experimental configuration
flag.
It includes a background update to backfill data, results from
the /threads endpoint will be partial until that finishes.
Applies the proper logic for unthreaded and threaded receipts to either
apply to all events in the room or only events in the same thread, respectively.
When retrieving counts of notifications segment the results based on the
thread ID, but choose whether to return them as individual threads or as
a single summed field by letting the client opt-in via a sync flag.
The summarization code is also updated to be per thread, instead of per
room.
c.f. #12993 (comment), point 3
This stores all device list updates that we receive while partial joins are ongoing, and processes them once we have the full state.
Note: We don't actually process the device lists in the same ways as if we weren't partially joined. Instead of updating the device list remote cache, we simply notify local users that a change in the remote user's devices has happened. I think this is safe as if the local user requests the keys for the remote user and we don't have them we'll simply fetch them as normal.
