gunixmounts: Make GUnixMountMonitor thread-safe
The Nautilus test suite often crashes with
GLib-FATAL-CRITICAL: g_source_is_destroyed: assertion 'g_atomic_int_get (&source->ref_count) > 0' failed if it is started with
GIO_USE_VOLUME_MONITOR=unix. This is because
GUnixMountMonitor is simultaneously used from multiple threads over
GVolumeMonitor APIs. Let's add guards for
mount_poller_time variables to prevent those crashes.
Fixes: #2030 (closed)