Don't delete persistent entries in DeleteAllEntries. (#71)

This is a behavior change but without it DeleteAllEntries is dangerous and
not very useful, so I consider the current behavior to be a bug.

src/Storage.h

@@ -164,6 +164,7 @@ class Storage {
       bool periodic,
       std::vector<std::pair<std::string, std::shared_ptr<Value>>>* entries)
       const;
+  void DeleteAllEntriesImpl();
 
   ATOMIC_STATIC_DECL(Storage)
 };