Previously a simple UPDATE query was used to backfill the table. This
causes excessive locking and is inefficient on large tables. Now we
instead run backfills in batches of a thousand rows. The batches are
determined based on primary key and iterated in ascending order.
Previously the new migrations were directly added to the migrations
lists. Now they are instead added to a vector in the InProgress status
and moved to the migrations list once the migration is complete.
Another status should be added later to indicate that a migration is
currently being applied. Right now, if a migration fails and then the
migrations are changed before being retried, there could be some
dangling changes.
