kv: import https://github.com/cznic/kv/commit/c721b822b5b1d882ade59203cdeedc136ab29573

Change-Id: Iff228f076abd5deacab1f8771edc9d87221a4dcc
Move osutil.Lock to its own package

Now in a small repo at github.com/camlistore/lock and brought into our third_party.

Change-Id: I966ca6126dde8b7c2e03f0d9fab1e36abd66c2ef