'Clear pdu_destination_tried cache'

def _clear_tried_cache(self):

    now = self._clock.time_msec()
    old_dict = self.pdu_destination_tried
    self.pdu_destination_tried = {}
    for (event_id, destination_dict) in old_dict.items():
        destination_dict = {dest: time for (dest, time) in destination_dict.items() if ((time + PDU_RETRY_TIME_MS) > now)}
        if destination_dict:
            self.pdu_destination_tried[event_id] = destination_dict
