Frontend CheckoutController #state_callbacks refactor

Hello there. This is just a suggestion for avoiding confusion among spree users such as the one described in #2488. ``state_callback(:after)`` doesn't make much sense and it doesn't behave as expected so I figured Spree may just be better off without it.

I also tried to clean up old invalid comments and spread ``load_order`` logic into more methods.

Please let me know if that looks good and I might submit patches for 1-2-stable as well.