ConcurrentQueue::nextJob: notify all _waitVar threads

Replace _waitVar.notify_one() with _waitVar.notify_all(). This was the
only hurdle to documenting ConcurrentQueue::waitAll() as thread-safe.

ConcurrentQueueTest::waitAllFromMultipleThreads() passes instead of
hanging now.

common/concurrent_queue.h

@@ -113,7 +113,7 @@ private:
                 --jobsLeft;
             }
 
-            _waitVar.notify_one();
+            _waitVar.notify_all();
         }
     }