Set thread_id column to non-null for event_push_{actions,actions_staging,summary} (#15350)

Clean-up from adding the thread_id column, which was initially
null but backfilled with values. It is desirable to require it to now
be non-null.

In addition to altering this column to be non-null, we clean up
obsolete background jobs, indexes, and just-in-time updating
code.