Remove the Sidekiq::Util dependency / Sidekiq 3.0.x compatibility

Sidekiq::Util is not intended for use by middleware.  Using it in middleware breaks in Sidekiq 3.0.  This PR addresses this issue by removing the dependency and implementing the required methods.

This restores compatibility with Sidekiq 3.0 (at least in specs) and maintains backwards compatibility with Sidekiq 2.14.0+, at the cost of a slight change in how the processor is displayed.  It may be desirable to restore the thread object id to the processor identifier.