Init signal

        3 signals are added: ``feeder_exited``, ``parser_exited`` and
        ``reach_max_num``.

def init_signal(self):
        """Init signal

        3 signals are added: ``feeder_exited``, ``parser_exited`` and
        ``reach_max_num``.
        """
        self.signal = Signal()
        self.signal.set(
            feeder_exited=False, parser_exited=False, reach_max_num=False)