AJ::Base.queue_adapter improvements

@cristianbica this is https://github.com/rails/rails/pull/16968
Sure, I needed this today in production actually. In the case of switching from one job queue to another, it may be desirable to enqueue a percentage of jobs to say, sidekiq, while the rest go to resque. We're running a similar thing right now where the percentage is stored in a redis key, allowing us to ramp up dynamically, and roll back if problems are detected (without a deploy).