Optimize away IOBuf clones on hot path

Summary:
'get' is the most frequent memcached operation. On a get reply, we should go to heap
as little as possible.  This diff optimizes for this scenario, where replies
have only one IOBuf field.

Reviewed By: pavlo-fb

Differential Revision: D3226592

fb-gh-sync-id: 92e1a1fac5735bd268691cf11990a96ae6fa8309
fbshipit-source-id: 92e1a1fac5735bd268691cf11990a96ae6fa8309

folly/io/Cursor.h

@@ -913,6 +913,10 @@ class QueueAppender : public detail::Writable<QueueAppender> {
     }
   }
 
+  void insert(const folly::IOBuf& buf) {
+    insert(buf.clone());
+  }
+
  private:
   folly::IOBufQueue* queue_;
   size_t growth_;