lock transactions: fix non-transactional writes

Lock transactions were actually writing *after* the lock was
released. The code was looking at the result of `release_write()` before
writing, then writing based on whether the lock was released.  This is
pretty obviously wrong.

- [x] Refactor `Lock` so that a release function can be passed to the
      `Lock` and called *only* when a lock is really released.

- [x] Refactor `LockTransaction` classes to use the release function
  instead of checking the return value of `release_read()` / `release_write()`