Don't drop in mixnet connection handlers (#2963)

mixnode/src/node/listener/connection_handler/mod.rs

@@ -77,6 +77,7 @@ impl ConnectionHandler {
         mut shutdown: TaskClient,
     ) {
         debug!("Starting connection handler for {:?}", remote);
+        shutdown.mark_as_success();
         let mut framed_conn = Framed::new(conn, SphinxCodec);
         while !shutdown.is_shutdown() {
             tokio::select! {