use correct generic type on AbstractDistributedValue.onChange

variables/src/main/java/io/atomix/variables/AbstractDistributedValue.java

@@ -54,7 +54,7 @@ protected AbstractDistributedValue(CopycatClient client, Properties options) {
    * @param callback The callback to be called when the value changes.
    * @return The change event.
    */
-  public synchronized CompletableFuture<Listener<ChangeEvent<T>>> onChange(Consumer<ChangeEvent<T>> callback) {
+  public synchronized CompletableFuture<Listener<ChangeEvent<U>>> onChange(Consumer<ChangeEvent<U>> callback) {
     return onEvent(Events.CHANGE, callback);
   }