class SRefCounted

 A reference-counted stone.
 Note that the ref-count is not serialized -- so rebuild it in
 post-streaming function when necessary.
 When zero ref count is reached in DecRefCount(),
   virtual void OnZeroRefCount();
 is called. Default action is to destroy the object. Note, mutex is NOT
 unlocked in this case ... so if you keep the object alive in OnZeroRefCnt()
 also unlock the mutex.