Session resource handling

[noxiouz]

To control session C pointer we must introduce destructor method, but we shouldn't dispose session until all callbacks are invoked (Seems it would lead to segfault).
To control proper context disposing it's better to associate Pool context map with a session. So if session.map is not empty - we cann't destroy and must wait.

@bioothod what do you think about it?

[bioothod]

Destructor method already exists, blocking until internal map of contexts is empty sounds like a good idea. It should also print error into log when destructor is called and yet there are some contexts

[bioothod]

Or even better - panic with stack trace if destructor has been called with non-empty context map