The original reason for the split was physical's dependencies, but those
haven't been onerous for a long time. Meanwhile it's a totally separate
implementation so we could be getting faulty results from tests. Get rid
of it and use the unified physical/inmem.
This:
* Allows removing LockingInmemStorage since the physical backend already
locks properly
* Makes listing work properly by adhering to expected semantics of only
listing up to the next prefix separator
* Reduces duplicated code
Jeff Mitchell