Refactor to the PulseAudio backend.

  * The main loop is now handled properly by pa_threaded_mainloop.
  * Rather than waiting for operations to complete inline, the main
    loop is now iterated on a separate thread.
  * Data is now written and read to and from the relevant stream via
    callbacks rather than a hacky loop.
  * Code overall has been simplified.
  * Includes a rant about bad API design in PulseAudio.

This should hopefully address these public issues:

  * https://github.com/mackron/miniaudio/issues/106
  * https://github.com/mackron/miniaudio/issues/187