Unsubscribe on delete

[jirenius]

Issue

When Resgate receives a delete event on any of its subscribed resources, it should unsubscribe any clients' direct subscriptions to that resource, including sending an unsubscribe event, after sending the delete event.

This is to avoid client to end up with conflicts between stale data of the deleted resource, and new data created with using same resource ID.

Notes

• The unsubscribe event should have the following reason:

  {
    "reason": {
        "code":"system.deleted",
        "message":"Deleted"
    }
  }

• Implementations where the client is set to call unsubscribe itself on delete events will get a system.noSubscription error as a response to their call.
• The unsubscribe prevents the client from keeping subscriptions of delete resources, but the service must still ensure no indirect subscriptions remain; it should remove all references to the resource ID before creating it anew.

[jirenius]

Resolved in PR #223