Add cache for ZDD nodes.
Introduce node cache in zdd node factory.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Refactor.
Avoid useless Long.valueOf.
Cache singletons.