Miscellaneous cleanup of load queue references (apache#16367)

Changes:
- Rename `DataSegmentChangeRequestAndStatus` to `DataSegmentChangeResponse`
- Rename `SegmentLoadDropHandler.Status` to `SegmentChangeStatus`
- Remove method `CoordinatorRunStats.getSnapshotAndReset()` as it was used only in
load queue peon implementations. Using an atomic reference is much simpler.
- Remove `ServerTestHelper.MAPPER`. Use existing `TestHelper.makeJsonMapper()` instead.

integration-tests/k8s/tiny-cluster.yaml

@@ -73,7 +73,6 @@ spec:
     druid.discovery.type=k8s
     druid.discovery.k8s.clusterIdentifier=druid-it
     druid.serverview.type=http
-    druid.coordinator.loadqueuepeon.type=http
     druid.indexer.runner.type=httpRemote
 
     # Metadata Store

server/src/main/java/org/apache/druid/server/coordination/DataSegmentChangeResponse.java

@@ -0,0 +1,64 @@
+/*
+ * 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.server.coordination;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+/**
+ * Response of a {@link DataSegmentChangeRequest}. Contains the request itself
+ * and the result {@link SegmentChangeStatus}.
+ */
+public class DataSegmentChangeResponse
+{
+  private final DataSegmentChangeRequest request;
+  private final SegmentChangeStatus status;
+
+  @JsonCreator
+  public DataSegmentChangeResponse(
+      @JsonProperty("request") DataSegmentChangeRequest request,
+      @JsonProperty("status") SegmentChangeStatus status
+  )
+  {
+    this.request = request;
+    this.status = status;
+  }
+
+  @JsonProperty
+  public DataSegmentChangeRequest getRequest()
+  {
+    return request;
+  }
+
+  @JsonProperty
+  public SegmentChangeStatus getStatus()
+  {
+    return status;
+  }
+
+  @Override
+  public String toString()
+  {
+    return "DataSegmentChangeResponse{" +
+           "request=" + request +
+           ", status=" + status +
+           '}';
+  }
+}

server/src/main/java/org/apache/druid/server/coordination/SegmentChangeStatus.java

@@ -0,0 +1,82 @@
+/*
+ * 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.server.coordination;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.common.base.Preconditions;
+
+import javax.annotation.Nullable;
+
+/**
+ * Contains {@link State} of a {@link DataSegmentChangeRequest} and failure
+ * message, if any.
+ */
+public class SegmentChangeStatus
+{
+  public enum State
+  {
+    SUCCESS, FAILED, PENDING
+  }
+
+  private final State state;
+  @Nullable
+  private final String failureCause;
+
+  public static final SegmentChangeStatus SUCCESS = new SegmentChangeStatus(State.SUCCESS, null);
+  public static final SegmentChangeStatus PENDING = new SegmentChangeStatus(State.PENDING, null);
+
+  public static SegmentChangeStatus failed(String cause)
+  {
+    return new SegmentChangeStatus(State.FAILED, cause);
+  }
+
+  @JsonCreator
+  private SegmentChangeStatus(
+      @JsonProperty("state") State state,
+      @JsonProperty("failureCause") @Nullable String failureCause
+  )
+  {
+    this.state = Preconditions.checkNotNull(state, "state must be non-null");
+    this.failureCause = failureCause;
+  }
+
+  @JsonProperty
+  public State getState()
+  {
+    return state;
+  }
+
+  @Nullable
+  @JsonProperty
+  public String getFailureCause()
+  {
+    return failureCause;
+  }
+
+  @Override
+  public String toString()
+  {
+    return "SegmentChangeStatus{" +
+           "state=" + state +
+           ", failureCause='" + failureCause + '\'' +
+           '}';
+  }
+}