An "entity change subscription"allows a client wiki to subscribe to changes to a given entity. These subscriptions would be used as the basis for dispatching changes. Currently, we dispatch changes based on sitelinks; this will no longer be sufficient with arbitrary access.
Note that we'll also need a script to populate the subscription table from a) the sitelinks table or better b) each client's entity_usage table. That should probably be a separate ticket though.
Whiteboard: u=dev c=backend p=8