Implement a simple blocking submit-poll-retrieve function

Sometimes, the whole parallelization scheme we have in place is an overkill, and all a user wants is to blockingly submit a problem and retrieve the result.

For this simple case we don't need thread pools and locks, making the `Client` trivially pickleable (#216, https://github.com/dwavesystems/dwave-system/issues/88), and consequently working with `multiprocessing` in process mode.