Starts sending periodic HeartBeat operations.

def start(self):
        """
        Starts sending periodic HeartBeat operations.
        """
        def _heartbeat():
            if not self._client.lifecycle.is_live:
                return
            self._heartbeat()
            self._heartbeat_timer = self._client.reactor.add_timer(self._heartbeat_interval, _heartbeat)

        self._heartbeat_timer = self._client.reactor.add_timer(self._heartbeat_interval, _heartbeat)