Potential deadlock if job state changes as graph finishes

The pause handler in Thor which utilizes a workunit watcher can deadlock, if it is notified at the same time it is stopping.
In which case the subscription layer, is withing a notify and Thor's handler is within an attempt to unsubscribe, but the notify cannot proceed as the handler requires a critical section that the stop() method where the unsubscribe is happening has.

Avoid by detaching the workunit watcher and releasing the crit before unsubscribe is called.