Switching to a sequence means there's no need to track `last_txn` on the
AS state table to generate new TXN IDs. This also means that there is
no longer contention between the AS scheduler and AS handler on updates
to the `application_services_state` table, which will prevent serialization
errors during the complete AS txn transaction.

Writing this column can be removed in a later release once the migration
to the sequence has been completed.

The sequence generators are already tested elsewhere with implementation
specifics for each DB type, so no point duplicating that here.