'A user has started syncing. Send a UserSync to the master, unless they
had recently stopped syncing.
Args:
user_id (str)'

def mark_as_coming_online(self, user_id):

    going_offline = self.users_going_offline.pop(user_id, None)
    if (not going_offline):
        self.send_user_sync(user_id, True, self.clock.time_msec())
