Make it possible to kill running sidekiq jobs

When we ran into gitlab-com/production#451, we couldn't find a way to kill the running sidekiq job.

Proposal

According to the Sidekiq FAQ you should write something like this:

class MyWorker
  include Sidekiq::Worker

  def perform(args)
    return if cancelled?
    # do stuff
  end

  def cancelled?
    Sidekiq.redis {|c| c.exists("cancelled-#{jid}") }
  end

  def self.cancel!(jid)
    Sidekiq.redis {|c| c.setex("cancelled-#{jid}", 86400, 1) }
  end
end

So in the implementation of workers, we should regularly check #cancelled? if we are allowed to continue the current worker.

For this we could create a helper something like:

def run_in_batches(relation, of: batch_size)
  relation.each_batch(of: batch_size) do |batch|
    break if cancelled?
    yield batch
  end
end