Set thread_id column to non-null for event_push_{actions,actions_staging,summary}

[clokep]

Updates the schema to require a thread_id (i.e. make the column non-null) for event_push_actions, event_push_actions_staging, and event_push_actions_summary.

This bumps the schema compat version to 74. This was last bumped in #15036, which was released in v1.77.0rc2 so users will be unable to rollback to earlier than that version.

This has two commits which are mostly separate, it performs the following operations:

• Deletes the background job to fill in any rows with thread_id = null in these tables.
• Completes the background job in the foreground. (We previously added an index in Update the thread_id right before use (in case the bg update hasn't finished) #14222 to make this fast.)
• Deletes the background jobs to create the indexes used above.
• (The following depend on Postgres vs. sqlite, but I'll write out the operations ignoring that sqlite needs to make a copy):
  • Drop the indexes used above.
  • Alter the thread_id column to be non-null.
• This causes a bunch of code to be unneeded, so we clean that up:
  • The background update to backfill this data.
  • The declarations to create the background indexes.
  • The just-in-time updating of the thread_id column (and the background checker to see if the background update is done).

Fixes #14225, reapplies part of #13776.

[clokep]

@erikjohnston I doubt you remember, but does this sound like what we were looking to do in #14225 / #14222? I think it is, but would be curious if you have concerns!

[reivilibre]

good otherwise