jack: Fix pipeline hang when jack changes sample rate or buffer size

jack: Fix pipeline hang when jack changes sample rate or buffer size

If jackd changes the buffer size or sample rate, jackaudiosink hangs
and can't be stopped. This also happens if jack is configured as slave
and a gstreamer pipeline is started on the slave machine while the jack
master isn't running yet. If the the jack master is started it changes
the buffer size / sample rate and jackaudiosink can't be stopped.

This fix calls jack_shutdown_cb when jack_sample_rate_cb or
jack_buffer_size_cb is called.

https://bugzilla.gnome.org/show_bug.cgi?id=771272