Add a convenience class for making REST API handlers retryable

We want to get all callers of BatchUpdate to use the new RetryHelper,
but explicitly passing a lambda to the RetryHelper instance in every
case is going to be a lot of boilerplate. Add some sugar for the most
common case, a REST API handler. Using the RetryingRestModifyView,
converting a handler to be retry-friendly is as simple as adding the
base class and changing a method name (and some minor cleanup like
removing a BatchUpdate.Factory field and twiddling exceptions).

Adding an intervening abstract class requires a tweak to the way
RestApiServlet extracts the input type using reflection. Fortunately,
there is a TypeLiteral method that does exactly what we want.

Change-Id: If2b78f0d556a49972ea9976057dd5f9dc21ec750

gerrit-server/src/main/java/com/google/gerrit/server/update/RetryingRestModifyView.java

@@ -0,0 +1,35 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.update;
+
+import com.google.gerrit.extensions.restapi.RestModifyView;
+import com.google.gerrit.extensions.restapi.RestResource;
+
+public abstract class RetryingRestModifyView<R extends RestResource, I, O>
+    implements RestModifyView<R, I> {
+  private final RetryHelper retryHelper;
+
+  protected RetryingRestModifyView(RetryHelper retryHelper) {
+    this.retryHelper = retryHelper;
+  }
+
+  @Override
+  public final O apply(R resource, I input) throws Exception {
+    return retryHelper.execute((updateFactory) -> applyImpl(updateFactory, resource, input));
+  }
+
+  protected abstract O applyImpl(BatchUpdate.Factory updateFactory, R resource, I input)
+      throws Exception;
+}