ChangeDispatcher currently needs to look up client subscriptions in two different ways: using wb_changes_subscription for clients with usage tracking, and using wb_item_per_site for clients without. This is inefficient and annoying. It would be much betetr to use the same mechansim for all client wikis.
Solution: roll out usage tracking for all client wikis. Arbitrary access need not be enabled for wikis where it is not yet wanted or possible.