Function handle_heartbeat 

async fn handle_heartbeat(
    state: &Arc<AppState>,
    clock: &ClockHandle,
    event_log_tx: &EventLogTx,
    rs: &mut ReducerState,
    host: &str,
    rollout_id: Option<RolloutId>,
    current_closure: Option<String>,
    at: DateTime<Utc>,
    reply: Sender<HeartbeatReply>,
)