Fix a close-time bug in the RtAudio Jack backend.

Can't see any reason why this call should be asynchronous, and it
causes (I think) a conflict between jack_deactivate() and
jack_client_close().
Fix a race condition during stopping of the RtAudio Jack back-end.

Race condition between calls to jack_deactivate() in the
jackStopStream asynchronous call and jack_client_close() in the main
thread.  Solution is to pthread_join() the jackStopStream thread
before calling jack_client_close().