A simple linear scaling strategy.

A slightly more advanced scaling strategy, when BinaryScalingStrategy isn't enough. LinearScalingStrategy#new takes two parameters: number of workers, and per-worker job capacity. Total work is the sum of the queued, scheduled, and retrying jobs. The strategy scales to the minimum amount of workers to handle all of the jobs. No more than the maximum amount of workers are ever scaled to. The default worker capacity is 25, Sidekiq's default concurrency.