Gracefully handle callback removal

BatchingJob auto-registers Sidekiq batch callbacks when you define an `on_x` method. The problem is that if you have a callback that you don't need anymore and update the code removing the method you'll get exceptions when the previously registered callbacks try to run. Since we're auto-registering it seems reasonable for us to also check that they are still there before trying to run them -- basically defining BatchingJob callbacks that no-op if the user's job hasn't implemented the callback methods. Some tricky async code change stuff to think through.