Fix Proxy slow disconnect.

When `select!` drops a JoinHandle, it doesn't abort the spawned task - it detaches it. From tokio docs:

A JoinHandle detaches the associated task when it is dropped, which means that there is no
longer any handle to the task, and no way to join on it.

This means the spawn task is still spinning in the background:
  1. c2s completes (client disconnected)
  2. select! drops the s2c JoinHandle
  3. The s2c task continues running in the background, detached
  4. That task holds the Mumble server connection open

The fix is to hand the async calls directly to select! instead of
calling tokio::spawn.

proxy/src/main.rs

@@ -335,19 +335,13 @@ async fn connect_proxy_impl(
 
     info!("connected to Mumble server");
 
-    // Spawn tasks to handle transmitting data between the WebTransport client and Mumble TCP Server
+    // Handle transmitting data between the WebTransport client and Mumble TCP Server
-    let c2s = tokio::spawn(
+    // When one direction completes/fails, the other is dropped and its streams are closed
-        pass_bytes_loop(incoming, write_server)
-            .instrument(info_span!("Handler", "Client to server")),
-    );
-    let s2c = tokio::spawn(
-        pass_bytes_loop(read_server, outgoing)
-            .instrument(info_span!("Handler", "Server to client")),
-    );
-
     tokio::select! {
-        res = c2s => res??,
+        res = pass_bytes_loop(incoming, write_server)
-        res = s2c => res??,
+            .instrument(info_span!("Handler", "Client to server")) => res?,
+        res = pass_bytes_loop(read_server, outgoing)
+            .instrument(info_span!("Handler", "Server to client")) => res?,
     };
     Ok(())
 }