There are cases when it would be very helpful if one could subscribe to the creations and invalidations of sessions.
- Clear parts of a cache when a session is no longer valid
- Show how many users are currently using the application
Something like a `SessionStateHub` could be added, that is triggered when a `SessionImpl` is created or invalidated. Then, there should be methods to register listeners, e.g. `addSessionCreatedListener`, `addSessionWillBeInvalidatedListener`, `addSessionInvalidatedListener`.