handles: use RegisteredPrefixHandle

refs #3919

Change-Id: I419363b279d7ac9a706c333ec1b4229c00965330
diff --git a/src/handles/read-handle.hpp b/src/handles/read-handle.hpp
index bdd6b6f..753e3d4 100644
--- a/src/handles/read-handle.hpp
+++ b/src/handles/read-handle.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2018, Regents of the University of California.
+ * Copyright (c) 2014-2019, Regents of the University of California.
  *
  * This file is part of NDN repo-ng (Next generation of NDN repository).
  * See AUTHORS.md for complete list of repo-ng authors and contributors.
@@ -37,7 +37,7 @@
   using DataPrefixUnregistrationCallback = std::function<void(const ndn::Name&)>;
   struct RegisteredDataPrefix
   {
-    const ndn::RegisteredPrefixId* prefixId;
+    ndn::RegisteredPrefixHandle hdl;
     int useCount;
   };