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

daemon/table/pit-entry.cpp

@@ -31,6 +31,7 @@ namespace pit {
 
 Entry::Entry(const Interest& interest)
   : m_interest(interest.shared_from_this())
+  , m_nameTreeEntry(nullptr)
 {
 }