sink,source: Avoid unnecessary call to pa_rtclock_now()

sink,source: Avoid unnecessary call to pa_rtclock_now()

pa_{sink,source}_volume_change_apply were being called by the ALSA I/O
thread on every iteration, causing a pa_rtclock_now() call, which can
sometimes be heavy. We avoid this call by making sure there actually are
changes to apply before proceeding into the function.

While we're at it, also dropping a redundant check on s->write_volume.