* Don't cache the old or active user to guarantee that SuState.auth_user
and SuState.active_user are always valid, even if ``request.user``
or ``request.session["su_auth_user_id"]`` changes. This also lazy-loads
SuState.old_user, which will improve performance for requests where it
isn't used.
* The internal API has been changed, hence the minor version bump. There
is no change to the external API.