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.
The return value can be used to make
ConcurrentQueueTest::cancelPending1UserThread() non-flaky. It may also
be useful to non-testing code.
Improve the performance of cancelPending() by locking the two mutexes
separately and minimizing the locking time.
Worker threads may well be executing jobs while this function is being
called. If ConcurrentQueue::waitAll() is called soon enough after
cancelPending(), the worker threads may still be running, but waitAll()
would return immediately as jobsLeft would be nonpositive.
Subtracting _queue.size() from jobsLeft sets this variable to the number
of worker threads that are executing jobs at the moment.
ConcurrentQueueTest::cancelPending1UserThread() passes most of the time
now. But it still fails occasionally because it depends on the timing of
thread scheduling, which is unreliable.
