Set thread_id column to non-null for event_push_{actions,actions_staging,summary} (#15437)

Updates the database schema to require a thread_id (by adding a
constraint that the column is non-null) for event_push_actions,
event_push_actions_staging, and event_push_actions_summary.

For PostgreSQL we add the constraint as NOT VALID, then
VALIDATE the constraint a background job to avoid locking
the table during an upgrade.

For SQLite we simply rebuild the table & copy the data.