Update event push action and receipt tables to support threads.

[clokep]

This is kind of split out of #13181 / #13202; it includes the database schema deltas only, with minor changes to the queries so we start inserting the correct values, and a background job to backfill data.

The overall design of this is to add a thread_id column to the event_push_actions (+ event_push_actions_staging) and event_push_summary tables. This allows the results to be segmented by the thread (by storing either the root event ID in the thread_id column or the special value "main").

receipts_linearized and receipts_graph also gain a thread_id column, which mostly works the same, but NULL is a special value meaning the receipt is "unthreaded".

See #13181 (comment)

After this we'll need to do a follow-up to actually use the new data we're inserting (and eventually remove the old indices).

For deployment / rollout purposes:

1. This adds the column(s) (and starts putting data into it).
2. PR2 will require thread_id on the event_push_* tables. (Which will require a schema bump?) At this point, we can do selects and joins properly for notification counts in threads. We'll drop the old indexes on (user_id, room_id).
3. PR3 will drop the constraints for receipts_* tables on (user_id, room_id, receipt_type) and update queries to actually use the thread_id field.

Part of #12550; implements the storage for MSC3771 and MSC3773.

[clokep]

Maybe we should be providing the thread_id like we do the room_id? (I.e. not storing it in event_push_actions_staging and only storing it in event_push_actions)

[clokep]

I think this needs to take into account the changes from #13703.