There appears to be a race where you can end up with entries in
`event_push_summary` with both a `NULL` and `main` thread ID.
Fixes#15736
Introduced in #15597
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.
Each table is updated as a separate schema delta to avoid
deadlocks between them.
For SQLite we simply rebuild the table & copy the data.
Erik Johnston