Punch through the storage.objstorage "collaborator" to get to the actual objstorages

This change introduced subtly confusing behavior, where the storage.objstorage
getattr proxy hides itself, until you try a dunder method (e.g. `foo in
storage.objstorage` or iter(storage.objstorage)), blows up in your face with a
confusing exception (because the collaborator is _also_ called ObjStorage).

Using the underlying objstorage works around that issue.