rewrite double linked list and hash table functions

[iHiroakiKawai]

Double linked list now uses a sentinel as a handler. If prev or next hits the sentinel, then those are the ends. The reason of using sentinel is that sentinel has a mutex internally, and could not be NULL. One more merit of using sentinel is performance improvement in scanning the elements from the last to first.

Hash table data structure became simple.

Memory usage was reduced.

[iHiroakiKawai]

I'll drop this request, because there're many things before rewriting this specific class.