[th/notify-queue] some optimization around g_object_freeze_notify()/g_object_thaw_notify()

• taking object references when unnecessary (in g_object_freeze_notify(), g_object_thaw_notify()).

• in g_object_thaw_notify(), avoid the freeze+thaw+thaw.

• in g_object_notify_by_spec_internal() path, take less notify_lock locks.