- Added new helper function :func:`.was_deleted`, returns True

- Added new helper function :func:`.was_deleted`, returns True
if the given object was the subject of a :meth:`.Session.delete`
operation.
- An object that's deleted from a session will be de-associated with
that session fully after the transaction is committed, that is
the :func:`.object_session` function will return None.
[ticket:2658]