cli: allow client process to control singleton process (#177141)

Other connected clients will now print:

```
Connected to an existing tunnel process running on this machine. You can press:

- Ctrl+C to detach
- "x" to stop the tunnel and exit
- "r" to restart the tunnel
```

These are then sent to the server to have that take effect. This is
mostly some refactors in the singleton_server to make the lifecycle work.

cli/src/util/sync.rs

@@ -35,6 +35,13 @@ where
 	}
 }
 
+#[async_trait]
+impl<T: Clone + Send + Sync> Receivable<T> for Barrier<T> {
+	async fn recv_msg(&mut self) -> Option<T> {
+		self.wait().await.ok()
+	}
+}
+
 #[derive(Clone)]
 pub struct BarrierOpener<T: Clone>(Arc<watch::Sender<Option<T>>>);