Speedup SearchResponse serialization (elastic#123211)

No need to have a nested concat here. There's obviously lots and lots of
room for optimization on this one, but just flattening out one obvious
step here outright halves the number of method calls required when
serializing a search response. Given that method calls can consume up to
half the serialization cost this change might massively speed up some
usecases.

Author: original-brownbear

server/src/main/java/org/elasticsearch/action/search/SearchResponse.java

@@ -14,13 +14,14 @@
 import org.elasticsearch.action.OriginalIndices;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.bytes.BytesReference;
+import org.elasticsearch.common.collect.Iterators;
 import org.elasticsearch.common.io.stream.DelayableWriteable;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
-import org.elasticsearch.common.xcontent.ChunkedToXContent;
+import org.elasticsearch.common.xcontent.ChunkedToXContentHelper;
 import org.elasticsearch.common.xcontent.ChunkedToXContentObject;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.RefCounted;
@@ -391,17 +392,24 @@ public Clusters getClusters() {
     @Override
     public Iterator<? extends ToXContent> toXContentChunked(ToXContent.Params params) {
         assert hasReferences();
-        return ChunkedToXContent.builder(params).xContentObject(innerToXContentChunked(params));
+        return getToXContentIterator(true, params);
     }
 
     public Iterator<? extends ToXContent> innerToXContentChunked(ToXContent.Params params) {
-        return ChunkedToXContent.builder(params)
-            .append(SearchResponse.this::headerToXContent)
-            .append(clusters)
-            .append(hits)
-            .appendIfPresent(aggregations)
-            .appendIfPresent(suggest)
-            .appendIfPresent(profileResults);
+        return getToXContentIterator(false, params);
+    }
+
+    private Iterator<ToXContent> getToXContentIterator(boolean wrapInObject, ToXContent.Params params) {
+        return Iterators.concat(
+            wrapInObject ? ChunkedToXContentHelper.startObject() : Collections.emptyIterator(),
+            ChunkedToXContentHelper.singleChunk(SearchResponse.this::headerToXContent),
+            Iterators.single(clusters),
+            hits.toXContentChunked(params),
+            aggregations == null ? Collections.emptyIterator() : ChunkedToXContentHelper.singleChunk(aggregations),
+            suggest == null ? Collections.emptyIterator() : ChunkedToXContentHelper.singleChunk(suggest),
+            profileResults == null ? Collections.emptyIterator() : ChunkedToXContentHelper.singleChunk(profileResults),
+            wrapInObject ? ChunkedToXContentHelper.endObject() : Collections.emptyIterator()
+        );
     }
 
     public XContentBuilder headerToXContent(XContentBuilder builder, ToXContent.Params params) throws IOException {