Function serve 

pub async fn serve(
    config: Config,
    factory: Factory,
    listener: TcpListener,
    shutdown: impl Future<Output = ()> + Send + 'static,
) -> Result<(), String>

Serves on listener until shutdown completes, then closes the WebSockets, releases every pressed button, stops the PLC workers (their queued writes still go out) and flushes the audit log.