txRDQ provides a ResizableDispatchQueue class that gives you a queue to
operate on a priority queue of jobs. Jobs can be canceled or reprioritized.
The size of the pool can be changed dynamically. Dispatching can be paused
and resumed. The queue can be stopped gracefully, and will return jobs that
had not yet been dispatched.  You can obtain the contents of the queue and
the dispatched pool while running.
