QueueingTaskDispatcher adds Tasks to a the back of a double ended queue
that is accessible to clients via the `pop_back` and `pop_front`
methods. Clients can manually take and run tasks, or call
`runLIFOUntilEmpty` or `runFIFOUntilEmpty` to run tasks until the queue
is empty.
QueueingTaskDispatcher is intended for use in unit tests, and as a
foundation for blocking convenience APIs in environments without
threads. QueueingTaskDispatcher should generally be avoided, and
ThreadPoolTaskDispatcher preferred, where threads are available.
