Function spawn_keep_alive_stream 

pub(crate) fn spawn_keep_alive_stream<S>(
    session_id: SessionId,
    sender: S,
    routing: DestinationRouting,
    notification_mode: SurbNotificationMode,
    cfg: Arc<BalancerStateValues>,
) -> (RateController, AbortHandle)
where
    S: Sink<(DestinationRouting, ApplicationDataOut)> + Clone + Send + Sync + Unpin + 'static,
    S::Error: Error + Send + Sync + 'static,

Spawns a task for a rate-limited stream of Keep-Alive messages to the Session counterparty.