Make scheduler more precise

[mperham]

I originally set the scheduler to fire within 15 seconds, because each Sidekiq process would check during that time and I didn't want someone with 100 Sidekiq processes slamming Redis.

Today the scheduler adapts for the number of processes in the cluster and randomly perturbs how often it checks to avoid overwhelming Redis; we should be able to safely lower it to something more precise.

I'd suggest we lower the average interval from 15 to 5 seconds.