Single event handler invocation queue per session

[yallie]

The current implementation uses a queue per subscription.
When a session initiates many subscriptions and doesn't unsubscribe
properly, it creates lots of work threads on the server.

Invocation queue is implemented as a thread pool.
Per-subscription queue is a pool that uses a single thread.
Per-session queue will probably need more than one thread (should be adjustable).

[yallie]

Implemented in df91c1a.