No way to unregister BB observer as a response to being notified about BB key change

Currently when a BB observer gets notified about BB key change there is not way to tell the BB component to unregister said observer.

The function called on observers should return a value indicating whether observer wants to continue observing or not (preferably an enum). Currently it returns void.

One thing to note here is if an observer tried to call "unregister observer" on "current" BB it could result in a crash since the external call comes from a const observers TMap iterator. Not sure if the behavior is defined in such a case.