Possibly fix lockfree hash table threading problems
- Fix initialization of memento manager link in getTileLazy. Created tile might fail to be linked to the data manager, if someone was quicker. Therefore we should link the tile to the history only after we confirmed the creation.
- Fix double-deletion in garbage collector. Now it uses a standard "raw-pointer users" approach with a simple spin-lock for a special case of too-many-objects-deleted.
- Add a unittest for getTileLazy algorithm.