Portability of `list-keys`

[lann]

I think we should consider moving list-keys into another/a new interface. Its doc comment already lists a number of caveats which reflect the underlying issue that this feature just isn't very portable.

/// MAY show an out-of-date list of keys if there are concurrent writes to the store.

I think this comment even undersells the problem somewhat; some KV stores (memcached, at least) can't even guarantee that you will see a key that wasn't updated during iteration.