There's a race condition in `remove()` as the lockfile is removed after
releasing the lock, which is a problem when another process acquires a
write lock during deletion.
Also simplify life a bit in multiprocessing when a file is possibly
removed multiple times, which currently is an error on the second
deletion, so the proposed fix is to make remove(...) idempotent and not
error when deleting non-existing cache entries.
Don't tests for existence of lockfile, cause windows/linux behavior is different
ea91e8f4ca