Provide an easy way to cancel a job without it being rescheduled

[robacarp]

For example:

class BetterSendItSoonJob < Mosquito::Job
  before do
    # only perform the job if this job was enqueued within the last 2 minutes
    fail "job is too stale to run", retry: false unless task.enqueue_time > 2.minutes.ago
  end

   def perform
      # ....
   end
end

Related #41