Release DocumentField instances in SearchHit.deallocate

These can consume non-trivial heap if transport messages start to queue
up. Lets release them asap.

Author: original-brownbear

server/src/main/java/org/elasticsearch/search/SearchHit.java

@@ -724,6 +724,14 @@ private void deallocate() {
             r.decRef();
         }
         SearchHit.this.source = null;
+        clearIfMutable(documentFields);
+        clearIfMutable(metaFields);
+    }
+
+    private static void clearIfMutable(Map<String, DocumentField> fields) {
+        if (fields instanceof HashMap<?, ?> hm) {
+            hm.clear();
+        }
     }
 
     @Override