- Query.UpdateDeleteTest.test_delete_fallback fails on mysql due to subquery in DELET...

- Query.UpdateDeleteTest.test_delete_fallback fails on mysql due to subquery in DELETE; not sure how to do this exact operation in MySQL
- added query_cls keyword argument to sessionmaker(); allows user-defined Query subclasses to be generated by query().
- added @attributes.on_reconstitute decorator, MapperExtension.on_reconstitute, both receieve 'on_load' attribute event allowing
non-__init__ dependent instance initialization routines.
- push memusage to the top to avoid pointless heisenbugs
- renamed '_foostate'/'_fooclass_manager' to '_sa_instance_state'/'_sa_class_manager'
- removed legacy instance ORM state accessors
- query._get() will use _remove_newly_deleted instead of expunge() on ObjectDeleted, so that transaction rollback
restores the previous state
- removed MapperExtension.get(); replaced by a user-defined Query subclass
- removed needless **kwargs from query.get()
- removed Session.get(cls, id); this is redundant against Session.query(cls).get(id)
- removed Query.load() and Session.load(); the use case for this method has never been clear, and the same functionality is available in more explicit ways