[AS7-370] Notification support

Update ModelControllerClient API so that registerNotificationHandler()
returns an object to unregister the handler (and the client no longer
has to keep a reference on them).
Fix the RegisterNotificationHandlerRequest to close the active
HandleNotificationRequest operation (instead of keeping it open until
the client is closed)
[AS7-370] Notification Support

use 2 different request handlers for registering/unregistering a
notification handler