Bootstrap polling for throttler.

        To avoid spiky traffic from throttler clients, we use a random delay
        before the first poll.

def _init_polling(self):
        """
        Bootstrap polling for throttler.

        To avoid spiky traffic from throttler clients, we use a random delay
        before the first poll.
        """
        with self.lock:
            if not self.running:
                return

            r = random.Random()
            delay = r.random() * self.refresh_interval
            self.channel.io_loop.call_later(
                delay=delay, callback=self._delayed_polling)
            self.logger.info(
                'Delaying throttling credit polling by %d sec', delay)