More flexible session detection

[d-maurer]

"plone.protect.auto.ProtectTransform._registered_objects" tries to detect session related objects based on the storage name ("temporary"). However, when it is important that sessions persist across server restarts or must be shared between several server instances, sessions may not be hosted by a storage named "temporary". It would be a good thing, if "plone.protect" would document that it is likely to fail when sessions are not within a storage named "temporary" and provide some way to prevent this failure.