Thread pool queue for the canceled subscriptions

[yallie]

How to reproduce:
https://gist.github.com/yallie/10a166266d5bab61675e1b226c99720f

1. Connect to server, attach an event handler to the remote event
2. Fire up a lot of events (like, a million)
3. Start receiving events from server
4. Kill the client
5. Observe the server cancelling broken subscriptions, as designed
6. All events enqueued before the subscription was canceled are still in the sending queue, and server tries to call canceled subscriptions anyway. Queued thread pool callbacks are processed very slowly because each one of them tries to send the event data to a disconnected client, times out, etc.

The server shouldn't try to route events for the canceled subscriptions.
Also, it should clean up the queued callbacks for the canceled subscriptions as fast as possible.