Implement Iterator

include/VersionedMap.h

@@ -135,6 +135,7 @@ struct VersionedMap {
     struct Impl;
 
   private:
+    friend struct VersionedMap;
     Impl *impl = nullptr;
   };
 
@@ -156,7 +157,7 @@ struct VersionedMap {
   Iterator begin(int64_t version) const;
 
   /** The "past-the-end" iterator. */
-  Iterator end() const;
+  Iterator end(int64_t version) const;
 
   /** Returns the memory usage in bytes. */
   int64_t getBytes() const;