It simplifies the interface of `bin/background_jobs` as:

1. start - Starting sidekiq or sidekiq-cluster on background.
2. restart - Kill all existing processes and start them
again (in background).
3. start_foreground - Starts sidekiq or sidekiq-cluster on foreground.
4. stop - Well, stops the process.

Here we also split the script into two `bin/background_jobs_sk` and
`bin/background_jobs_sk_cluster`. If `SIDEKIQ_WORKERS` env var is
given we use the `bin/background_jobs_sk_cluster`, which has
basically the same API of the sidekiq counterpart.