Enhanced Sidekiq and SuckerPunch configuration

This is a refactor exercice around `Rollbar::Configuration#use_sidekiq=` and `Rollbar::Configuration#use_sucker_punch=` methods which has led to a much better isolated classes `Rollbar::Delay::Sidekiq` and `Rollbar::Delay::SuckerPunch`.
Those classes instance variables responds to #call which allows to mock them for proc or lambda and behave like an ordinary `Rollbar::Configuration#async_handler`.