Add a QThreadPoolExecutor which uses a QThreadPool object

[kristjanvalur]

It can be useful, when using loop.run_in_scheduler(), to simply use the pre-existing pool of QThread objects.
A global instance of QThreadPool exists in the application, and so this is typically faster than creating a QThreadExecutor instance, with a fixed number of new threads.

• A new QThreadPoolExecutor class is created. It can take a QThreadPool object as the underlying pool or use the global thread pool otherwise.
• A QThreadExecutorBase class becomes the parent of both QThreadExecutor and QThreadPoolExecutor.
• add the required map() method, using the underlying submit().
• Update signature of the shutdown() method, supporting the cancel_futures argument.
• Add various unit tests.