Speed up synthetic source again

When I added support for stored fields to synthetic _source (elastic#87480) I
accidentally caused a performance regression. Our friends working on
building the nightly charts for tsdb caught it. It looked like:
```
|   50th percentile latency | default_1k | 20.1228 | 41.289  | 21.1662  | ms | +105.18% |
|   90th percentile latency | default_1k | 26.7402 | 42.5878 | 15.8476  | ms |  +59.27% |
|   99th percentile latency | default_1k | 37.0881 | 45.586  |  8.49786 | ms |  +22.91% |
| 99.9th percentile latency | default_1k | 43.7346 | 48.222  |  4.48742 | ms |  +10.26% |
|  100th percentile latency | default_1k | 46.057  | 56.8676 | 10.8106  | ms |  +23.47% |
```

This fixes the regression and puts us in line with how we were:
```
|   50th percentile latency | default_1k | 20.1228 | 24.023  |  3.90022 | ms |  +19.38% |
|   90th percentile latency | default_1k | 26.7402 | 29.7841 |  3.04392 | ms |  +11.38% |
|   99th percentile latency | default_1k | 37.0881 | 36.8038 | -0.28428 | ms |   -0.77% |
| 99.9th percentile latency | default_1k | 43.7346 | 39.0192 | -4.71531 | ms |  -10.78% |
|  100th percentile latency | default_1k | 46.057  | 42.9181 | -3.13889 | ms |   -6.82% |
```

A 20% bump in the 50% latency isn't great, but it four microseconds per
document which is acceptable.

server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java

@@ -604,7 +604,21 @@ public DocValuesLoader docValuesLoader(LeafReader leafReader, int[] docIdsInLeaf
                     loaders.add(loader);
                 }
             }
-            return docId -> {
+            if (loaders.isEmpty()) {
+                return null;
+            }
+            return new ObjectDocValuesLoader(loaders);
+        }
+
+        private class ObjectDocValuesLoader implements DocValuesLoader {
+            private final List<SourceLoader.SyntheticFieldLoader.DocValuesLoader> loaders;
+
+            private ObjectDocValuesLoader(List<DocValuesLoader> loaders) {
+                this.loaders = loaders;
+            }
+
+            @Override
+            public boolean advanceToDoc(int docId) throws IOException {
                 for (SourceLoader.SyntheticFieldLoader.DocValuesLoader docValueLoader : loaders) {
                     boolean leafHasValue = docValueLoader.advanceToDoc(docId);
                     hasValue |= leafHasValue;
@@ -616,7 +630,7 @@ public DocValuesLoader docValuesLoader(LeafReader leafReader, int[] docIdsInLeaf
                  * stored field.
                  */
                 return hasValue;
-            };
+            }
         }
 
         @Override

server/src/main/java/org/elasticsearch/index/mapper/SourceLoader.java

@@ -106,8 +106,18 @@ public Set<String> requiredStoredFields() {
 
         @Override
         public Leaf leaf(LeafReader reader, int[] docIdsInLeaf) throws IOException {
-            SyntheticFieldLoader.DocValuesLoader leaf = loader.docValuesLoader(reader, docIdsInLeaf);
-            return (fieldsVisitor, docId) -> {
+            return new SyntheticLeaf(loader.docValuesLoader(reader, docIdsInLeaf));
+        }
+
+        private class SyntheticLeaf implements Leaf {
+            private final SyntheticFieldLoader.DocValuesLoader docValuesLoader;
+
+            private SyntheticLeaf(SyntheticFieldLoader.DocValuesLoader docValuesLoader) {
+                this.docValuesLoader = docValuesLoader;
+            }
+
+            @Override
+            public BytesReference source(FieldsVisitor fieldsVisitor, int docId) throws IOException {
                 if (fieldsVisitor != null) {
                     for (Map.Entry<String, List<Object>> e : fieldsVisitor.fields().entrySet()) {
                         SyntheticFieldLoader.StoredFieldLoader loader = storedFieldLoaders.get(e.getKey());
@@ -118,14 +128,14 @@ public Leaf leaf(LeafReader reader, int[] docIdsInLeaf) throws IOException {
                 }
                 // TODO accept a requested xcontent type
                 try (XContentBuilder b = new XContentBuilder(JsonXContent.jsonXContent, new ByteArrayOutputStream())) {
-                    if (leaf.advanceToDoc(docId)) {
+                    if (docValuesLoader.advanceToDoc(docId)) {
                         loader.write(b);
                     } else {
                         b.startObject().endObject();
                     }
                     return BytesReference.bytes(b);
                 }
-            };
+            }
         }
     }