def service_timeouts(self):
        """
        run callbacks on all expired timers
        Called from the event thread
        :return: next end time, or None
        """
        queue = self._queue
        if self._new_timers:
            new_timers = self._new_timers
            while new_timers:
                heappush(queue, new_timers.pop())

        if queue:
            now = time.time()
            while queue:
                try:
                    timer = queue[0][1]
                    if timer.finish(now):
                        heappop(queue)
                    else:
                        return timer.end
                except Exception:
                    log.exception("Exception while servicing timeout callback: ")
run callbacks on all expired timers
        Called from the event thread
        :return: next end time, or None