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.
One other behaviour this is missing is that when `#find` is called with an unknown id it will raise an `ActiveRecord::RecordNotFound` exception. I would suggest updating the method to do this as well.