Make bin/background_jobs use Sidekiq cluster

What does this MR do?

Sets the default number of workers to 1 if SIDEKIQ_WORKERS is not provided.

This also fixes a bug where we are starting 1 more process than requested.

This prepares us for the upgrade to Sidekiq 6 since sidekiqctl and other daemonization features have been removed.

How to setup and validate locally (strongly suggested)

Run bin/background_jobs and see that it works normally