use active job for creating stock items

This one is going to break some tests :)

Here, I've moved stock item creation out into a job. This is because it takes, on my development machine, ~2 minutes to complete.

Version 0 (Rails 4.1 compatible) uses `enqueue` to perform jobs. I am currently using version 0 of Active Job with Spree 2.4 and its working fine. In Rails 4.2, the method for adding jobs to the queue was renamed to `perform_later`.

I think there's a lot of work to be done, but I feel like this is the first step in trying to get AJ into Spree for performance reasons.

I'm thinking of having a `defined? ActiveJob` check so that users can cherry-pick this code into Spree 2.4.

Thoughts?
What is this supposed to be doing?

Whatever the reason this is here it's probably not working. Jobs may be run in different processes or different servers. A mutex will only work across threads.