make utf8 bytes response not reuse thread local buffer

no need, optimized conversion to bytes anyhow, and when sending, it will just get wrapped by a buffer

src/main/java/org/elasticsearch/rest/StringRestResponse.java

@@ -27,13 +27,6 @@ import org.apache.lucene.util.UnicodeUtil;
  */
 public class StringRestResponse extends Utf8RestResponse {
 
-    private static ThreadLocal<BytesRef> cache = new ThreadLocal<BytesRef>() {
-        @Override
-        protected BytesRef initialValue() {
-            return new BytesRef();
-        }
-    };
-
     public StringRestResponse(RestStatus status) {
         super(status);
     }
@@ -43,7 +36,7 @@ public class StringRestResponse extends Utf8RestResponse {
     }
 
     private static BytesRef convert(String content) {
-        BytesRef result = cache.get();
+        BytesRef result = new BytesRef();
         UnicodeUtil.UTF16toUTF8(content, 0, content.length(), result);
         return result;
     }

src/main/java/org/elasticsearch/rest/Utf8RestResponse.java

@@ -25,8 +25,6 @@ import org.apache.lucene.util.BytesRef;
  * An http response that is built on top of {@link org.apache.lucene.util.BytesRef}.
  * <p/>
  * <p>Note, this class assumes that the utf8 result is not thread safe.
- *
- *
  */
 public class Utf8RestResponse extends AbstractRestResponse implements RestResponse {
 
@@ -58,7 +56,7 @@ public class Utf8RestResponse extends AbstractRestResponse implements RestRespon
 
     @Override
     public boolean contentThreadSafe() {
-        return false;
+        return true;
     }
 
     @Override