Deduplicate Spree::Adjustment loads for performance

* Reduces the available object graph paths to access Spree::Adjustment
  instances to one.
* Reduces the amount of duplicates being loaded via performing in-memory
  scoping on associations.
* Significantly reduces the amount of lazy loads triggered within most
  order operations / populations through the elimination of duplicates.
As much as I like that you were modifying double quotes to single quotes, I don't see any other modifications to this code. My assumption was that you were going to limit style cleanups to just code that you are already touching.