Extract method object: Resque::JobPerformer.

This is the first step of a refactoring. As you can see, tons of
dependencies here. Time to untangle them.
let's wrap those hooks up into one structure
Various fixes and changes to Config and its tests

* We were loading up stuff from the legacy suite. I don't want
  to do that.
* We don't have ActiveSupport as a dependency, but it's a dependency of
  one of our tests. Because of the above issue, the code was written
  using an AS method that isn't actually available outside of the tests.
* We want to `ensure` that our env is fixed. Previously, if a test had
  failed, our `ENV` was still modified, which causes unrelated tests to
  fail.
* Use Kernel.warn rather than puts to warn.
* Warnings can now be suppressed with the `suppress_warnings` method.
* With all of this, tests are now more robust and also run with no
  extra output. :metal: