Currently GenericTaskQueueSet::steal() records only the amount of times the method is called and a false (unable to steal) value is returned.

Internally it actually attempts to steal from other queues many times. This gives a wrong impression on how the actual ratio between steal attempts and successful steals is.

Since the number of times to call the method is not that interesting (and it roughly corresponds to already measured number of times we offer termination), change it to record actual steal attempts.