Watch packet list for timeouts.

def _packet_manager(self):
        """ Watch packet list for timeouts. """
        while True:
            if self._packets:
                with self._packet_lock:
                    now = time.time()

                    self._packets[:] = \
                        [packet for packet in self._packets
                         if self._packet_timeout(packet, now)]

            # c.f. nyquist
            time.sleep(ACK_RESEND / 2)