add get_message_subsystem() function

src/net/channel.rs

@@ -174,6 +174,10 @@ impl Channel {
             .await;
     }
 
+    pub fn get_message_subsystem(&self) -> &MessageSubsystem {
+        &self.message_subsystem
+    }
+
     async fn main_receive_loop(self: Arc<Self>) -> NetResult<()> {
         debug!(target: "net",
             "Channel::receive_loop() [START, address={}]",