Follow up on adjustment refactoring latest changes

I'm creating this just as a way to tell you where I'm going with adjustments refactoring so that once you get back you can have a good idea of what I've done. Later on I might create separate PRs for each change as we all agree which ones are good. And in case this all sounds good I can clear the not appreciated merge commits and do some more squash so that it can be merged the spree way.

btw this already includes #2, #3, #4, #5

Let me know your thoughts once you have a chance :) I should keep adding more commits over the week right now I'm looking at how the store behaves going through frontend checkout.