- session checks more carefully when determining "object X already in another session";

- session checks more carefully when determining "object X already in another session";
e.g. if you pickle a series of objects and unpickle (i.e. as in a Pylons HTTP session
or similar), they can go into a new session without any conflict
- added stricter checks around session.delete() similar to update()
- shored up some old "validate" stuff in session/uow