Ohm::Locking: bugfix and rename

- Rename Ohm::Locking#mutex to Ohm::Locking#spinlock
  - Yes I admit the naming has some coincidence with the HN
    thread involving linus here:

    http://yarchive.net/comp/linux/semaphores.html

  - More importantly, the rename has something to do with the
    fact that `Ohm::Model.mutex` now exists, so trying to
    extend `Ohm::Locking` into a model will result in a
    conflict.

- Fix the bug
  - So apparently, no one cares about Ohm::Locking anymore,
    and I sort of fell off the grid so I never bothered to look.
  - The bug involves not checking the value of SETNX explicitly
    as 1 / 0, but just doing the truthy check (compliant with the
    redis-rb protocol)
  - Dunno why this is using HSET instead of SET as we're dealing
    with a direct key here.

- Don't return self, as I don't remember why we even did that
  in the past. It's surprising and more often is not the
  behavior we're looking for.