Create `WorkerOld`, copy of `Worker`

This is setup for changing `Worker` to work with the new `Server`.
This takes the current worker logic and moves it to `WorkerOld`
and updates everything using `Worker` to use `WorkerOld` instead.

I did this to free-up the `Worker` name but I also wanted to
preserve the diff from the old worker logic to the new worker
logic which will come in a separate commit. That's why I left the
old worker logic in `sanford/worker.rb`.

@kellyredding - Ready for review. I'm not changing any functionality here, just prepping for the changes to `Worker` to work with the new `Server` but keep the old stuff intact. There aren't many changes to `Worker`, so I wanted to highlight them in the diff.