[SPARK-5414] Add SparkFirehoseListener class for consuming all SparkListener events

There isn't a good way to write a SparkListener that receives all SparkListener events and which will be future-compatible (e.g. it will receive events introduced in newer versions of Spark without having to override new methods to process those events).

To address this, this patch adds `SparkFirehoseListener`, a SparkListener implementation that receives all events and dispatches them to a single `onEvent` method (which can be overridden by users).

Author: Josh Rosen <joshrosen@databricks.com>

Closes #4210 from JoshRosen/firehose-listener and squashes the following commits:

223f579 [Josh Rosen] Expand comment to explain rationale for this being a Java class.
ecdfaed [Josh Rosen] Add SparkFirehoseListener class for consuming all SparkListener events.

core/src/main/java/org/apache/spark/SparkFirehoseListener.java

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.spark;
+
+import org.apache.spark.scheduler.*;
+
+/**
+ * Class that allows users to receive all SparkListener events.
+ * Users should override the onEvent method.
+ *
+ * This is a concrete Java class in order to ensure that we don't forget to update it when adding
+ * new methods to SparkListener: forgetting to add a method will result in a compilation error (if
+ * this was a concrete Scala class, default implementations of new event handlers would be inherited
+ * from the SparkListener trait).
+ */
+public class SparkFirehoseListener implements SparkListener {
+
+    public void onEvent(SparkListenerEvent event) { }
+
+    @Override
+    public final void onStageCompleted(SparkListenerStageCompleted stageCompleted) {
+        onEvent(stageCompleted);
+    }
+
+    @Override
+    public final void onStageSubmitted(SparkListenerStageSubmitted stageSubmitted) {
+        onEvent(stageSubmitted);
+    }
+
+    @Override
+    public final void onTaskStart(SparkListenerTaskStart taskStart) {
+        onEvent(taskStart);
+    }
+
+    @Override
+    public final void onTaskGettingResult(SparkListenerTaskGettingResult taskGettingResult) {
+        onEvent(taskGettingResult);
+    }
+
+    @Override
+    public final void onTaskEnd(SparkListenerTaskEnd taskEnd) {
+        onEvent(taskEnd);
+    }
+
+    @Override
+    public final void onJobStart(SparkListenerJobStart jobStart) {
+        onEvent(jobStart);
+    }
+
+    @Override
+    public final void onJobEnd(SparkListenerJobEnd jobEnd) {
+        onEvent(jobEnd);
+    }
+
+    @Override
+    public final void onEnvironmentUpdate(SparkListenerEnvironmentUpdate environmentUpdate) {
+        onEvent(environmentUpdate);
+    }
+
+    @Override
+    public final void onBlockManagerAdded(SparkListenerBlockManagerAdded blockManagerAdded) {
+        onEvent(blockManagerAdded);
+    }
+
+    @Override
+    public final void onBlockManagerRemoved(SparkListenerBlockManagerRemoved blockManagerRemoved) {
+        onEvent(blockManagerRemoved);
+    }
+
+    @Override
+    public final void onUnpersistRDD(SparkListenerUnpersistRDD unpersistRDD) {
+        onEvent(unpersistRDD);
+    }
+
+    @Override
+    public final void onApplicationStart(SparkListenerApplicationStart applicationStart) {
+        onEvent(applicationStart);
+    }
+
+    @Override
+    public final void onApplicationEnd(SparkListenerApplicationEnd applicationEnd) {
+        onEvent(applicationEnd);
+    }
+
+    @Override
+    public final void onExecutorMetricsUpdate(
+            SparkListenerExecutorMetricsUpdate executorMetricsUpdate) {
+        onEvent(executorMetricsUpdate);
+    }
+
+    @Override
+    public final void onExecutorAdded(SparkListenerExecutorAdded executorAdded) {
+        onEvent(executorAdded);
+    }
+
+    @Override
+    public final void onExecutorRemoved(SparkListenerExecutorRemoved executorRemoved) {
+        onEvent(executorRemoved);
+    }
+}