Add test cases for executing Session operations within call/event slots

[ecorm]

Add test cases for Session operations being executed within call/event slots. For example, verify that a client app can unsubscribe from within a pub/sub event handler.

Event/call slots are always posted to the I/O service; they are never executed with the context of the WAMP message receive handler. So while I doubt there are currently are any issues, it would be good to test for them anyway.