added a toString for tracing

Author: robertroeser

src/main/java/io/reactivesocket/DefaultReactiveSocket.java

@@ -492,4 +492,8 @@ public void cancel() {
             });
         };
     }
+
+    public String toString() {
+        return "duplexConnection=[" + this.connection + "]";
+    }
 }