Experiment with IteratingCallback

The previous semantic of `onCompleteFailure` has been renamed to `onFailure(Throwable)`, which is called immediately (but serialized) on either an abort or a failure.   A new `onCompleteFailure(Throwable)` method has been added that is called only after a `failed(throwable)` or a `abort(Throwable)` followed by `succeeded()` or `failed(Throwable)``

No usage has yet been made of the new `onCompleteFailure`, but the ICB implementation has been completely replaced by the one developed in #11876

jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/IteratingCallback.java

@@ -44,18 +44,13 @@
  * <p>
  * Subclasses must implement method {@link #process()} where the
  * asynchronous sub-task is initiated and a suitable {@link Action}
- * is returned to this callback to indicate the overall progress ofk
+ * is returned to this callback to indicate the overall progress of
  * the large asynchronous task.
  * This callback is passed to the asynchronous sub-task, and a call
  * to {@link #succeeded()} on this callback represents the successful
  * completion of the asynchronous sub-task, while a call to
  * {@link #failed(Throwable)} on this callback represents the
  * completion with a failure of the large asynchronous task.
- * <p>
- * For most purposes, the {@link #succeeded()} and {@link #failed(Throwable)}
- * methods of this class should be considered final, and only overridden in
- * extraordinary circumstances. Any action taken in such extensions are not
- * serialized.
  */
 public abstract class IteratingCallback implements Callback
 {

jetty-ee11/jetty-ee11-proxy/src/main/java/org/eclipse/jetty/ee11/proxy/AsyncProxyServlet.java

@@ -192,9 +192,9 @@ protected void onRequestContent(HttpServletRequest request, Request proxyRequest
         }
 
         @Override
-        public void failed(Throwable x)
+        public void onFailure(Throwable x)
         {
-            super.failed(x);
+            super.onFailure(x);
             onError(x);
         }
     }