Decouple LimitedCapacity::Worker from Sidekiq queue size

As described in #995 (closed), the LimitedCapacity::Worker Rails concern relies on Sidekiq queue size to manage job scheduling. The underlying assumption is that each Sidekiq worker has a dedicated queue which is true today but which won't be true in the future.

This issue is for tracking the work to decouple LimitedCapacity::Worker from Sidekiq queue size.