table: make name_tree::Node the sole owner of name_tree::Entry

This commit also improves name_tree::Entry test coverage.

refs #3687

Change-Id: I92375f29fbebab82c67da7dff2ea7a2952ba2cca
diff --git a/daemon/table/name-tree-iterator.hpp b/daemon/table/name-tree-iterator.hpp
index f484440..ecde5bd 100644
--- a/daemon/table/name-tree-iterator.hpp
+++ b/daemon/table/name-tree-iterator.hpp
@@ -26,11 +26,13 @@
 #ifndef NFD_DAEMON_TABLE_NAME_TREE_ITERATOR_HPP
 #define NFD_DAEMON_TABLE_NAME_TREE_ITERATOR_HPP
 
-#include "name-tree-entry.hpp"
+#include "name-tree-hashtable.hpp"
 
 namespace nfd {
 namespace name_tree {
 
+class NameTree;
+
 /** \brief a predicate to accept or reject an Entry in find operations
  */
 typedef function<bool(const Entry& entry)> EntrySelector;