JobQueue: fixed leak, notification, added Pop() variant.

* Was leaking fQueuedJobs on destruction.
* fHaveRunnableJobSem implementation was not completed; it was never
  released.
* Added Pop() variant that is a bit more flexible, and allows for a
  timeout as well as waiting even when the queue is empty, and can
  return a status code.

headers/private/support/JobQueue.h

@@ -30,8 +30,12 @@ public:
 									// gives up ownership
 
 			BJob*				Pop();
+			status_t			Pop(bigtime_t timeout, bool returnWhenEmpty,
+									BJob** _job);
 									// caller owns job
 
+			size_t				CountJobs() const;
+
 			void				Close();
 
 private:
@@ -49,7 +53,7 @@ private:
 			void				_RequeueDependantJobsOf(BJob* job);
 			void				_RemoveDependantJobsOf(BJob* job);
 
-			BLocker				fLock;
+	mutable	BLocker				fLock;
 			uint32				fNextTicketNumber;
 			JobPriorityQueue*	fQueuedJobs;
 			sem_id				fHaveRunnableJobSem;