MSQ worker: Support in-memory shuffles.

This patch is a follow-up to apache#16168, adding worker-side support for
in-memory shuffles. Changes include:

1) Worker-side code now respects the same context parameter "maxConcurrentStages"
   that was added to the controller in apache#16168. The parameter remains undocumented
   for now, to give us a chance to more fully develop and test this functionality.

1) WorkerImpl is broken up into WorkerImpl, RunWorkOrder, and RunWorkOrderListener
   to improve readability.

2) WorkerImpl has a new StageOutputHolder + StageOutputReader concept, which
   abstract over memory-based or file-based stage results.

3) RunWorkOrder is updated to create in-memory stage output channels when
   instructed to.

4) ControllerResource is updated to add /doneReadingInput/, so the controller
   can tell when workers that sort, but do not gather statistics, are done reading
   their inputs.

5) WorkerMemoryParameters is updated to consider maxConcurrentStages.

Additionally, WorkerChatHandler is split into WorkerResource, so as to match
ControllerChatHandler and ControllerResource.

extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/exec/ControllerClient.java

@@ -26,17 +26,21 @@
 import org.apache.druid.msq.statistics.PartialKeyStatisticsInformation;
 
 import javax.annotation.Nullable;
+import java.io.Closeable;
 import java.io.IOException;
 import java.util.List;
 
 /**
- * Client for the multi-stage query controller. Used by a Worker task.
+ * Client for the multi-stage query controller. Used by a {@link Worker}. Each instance is specific to a single query,
+ * meaning it communicates with a single controller.
  */
-public interface ControllerClient extends AutoCloseable
+public interface ControllerClient extends Closeable
 {
   /**
-   * Client side method to update the controller with partial key statistics information for a particular stage and worker.
-   * Controller's implementation collates all the information for a stage to fetch key statistics from workers.
+   * Client side method to update the controller with partial key statistics information for a particular stage
+   * and worker. The controller collates all the information for a stage to fetch key statistics from workers.
+   *
+   * Only used when {@link StageDefinition#mustGatherResultKeyStatistics()}.
    */
   void postPartialKeyStatistics(
       StageId stageId,
@@ -77,20 +81,21 @@ void postResultsComplete(
 
   /**
    * Client side method to inform the controller that the error has occured in the given worker.
+   *
+   * @param queryId      query ID, if this error is associated with a specific query
+   * @param errorWrapper error details
    */
   void postWorkerError(
-      String workerId,
+      @Nullable String queryId,
       MSQErrorReport errorWrapper
   ) throws IOException;
 
   /**
    * Client side method to inform the controller about the warnings generated by the given worker.
    */
-  void postWorkerWarning(
-      List<MSQErrorReport> MSQErrorReports
-  ) throws IOException;
+  void postWorkerWarning(List<MSQErrorReport> MSQErrorReports) throws IOException;
 
-  List<String> getTaskList() throws IOException;
+  List<String> getWorkerIds() throws IOException;
 
   /**
    * Close this client. Idempotent.

extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/exec/ListeningOutputChannelFactory.java

@@ -0,0 +1,74 @@
+/*
+ * 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.druid.msq.exec;
+
+import org.apache.druid.frame.processor.OutputChannel;
+import org.apache.druid.frame.processor.OutputChannelFactory;
+import org.apache.druid.frame.processor.PartitionedOutputChannel;
+
+import java.io.IOException;
+
+/**
+ * Decorator for {@link OutputChannelFactory} that notifies a {@link Listener} whenever a channel is opened.
+ */
+public class ListeningOutputChannelFactory implements OutputChannelFactory
+{
+  private final OutputChannelFactory delegate;
+  private final Listener listener;
+
+  public ListeningOutputChannelFactory(final OutputChannelFactory delegate, final Listener listener)
+  {
+    this.delegate = delegate;
+    this.listener = listener;
+  }
+
+  @Override
+  public OutputChannel openChannel(final int partitionNumber) throws IOException
+  {
+    return notifyListener(delegate.openChannel(partitionNumber));
+  }
+
+
+  @Override
+  public OutputChannel openNilChannel(final int partitionNumber)
+  {
+    return notifyListener(delegate.openNilChannel(partitionNumber));
+  }
+
+  @Override
+  public PartitionedOutputChannel openPartitionedChannel(
+      final String name,
+      final boolean deleteAfterRead
+  )
+  {
+    throw new UnsupportedOperationException("Listening to partitioned channels is not supported");
+  }
+
+  private OutputChannel notifyListener(OutputChannel channel)
+  {
+    listener.channelOpened(channel);
+    return channel;
+  }
+
+  public interface Listener
+  {
+    void channelOpened(OutputChannel channel);
+  }
+}