Speed up the database by caching the Occurrence hash.
* Change: In Hurricane::SharedPath, the hash for the shared pathes, which serves for Map<> storing, were computed "on the fly" at each ::getHash() call, and were involving recursive calls of all the hashes along the components of the shared path. This is terribly slow especially in a design with a deep hierarchy (typical case LS180). Now, at the cost of one supplemenental unsigned long in each SharedPath, it is only computed once in the constructor, and is no longer recursive (only access the *first* tail, if any). As a consequence, the re-display computation delay becomes bearable. But also speed up any transhierarchical walkthough.