os: add prototype KineticStore

Implement the KeyValueDB interface using libkinetic_client,
and allow it to be configured as the backend for the KeyValueStore,
running the entire OSD on it.

This prototype implementation has no transaction safety, and is
only suitable as a proof of concept. Since the libkinetic_client
API does not provide reverse iteration over keys without also reading
the value off disk, it implements iterators in a very slow but correct way.
These are used heavily by the KeyValueDB callers, so this is a bottleneck
in performance.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>