Closing mdns agent

I use the library with many open/close, and I get mdns storms where the webrtc library searches for other nodes over the network in a very aggressive manner.

This has been proposed by @r-byondlabs, and might very well be the problem: if I open/close many connections, and the mdns-agents are not cleaned up, there might be many requests.

Fixes #616

ice/src/agent/mod.rs

@@ -349,6 +349,8 @@ impl Agent {
             udp_mux.remove_conn_by_ufrag(&ufrag).await;
         }
 
+        self.close_multicast_conn(self.mdns_conn).await;
+
         //FIXME: deadlock here
         self.internal.close().await
     }