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.
@dkubb This was me with a failed regexp grepping for touched but non converted quotes from the `git diff` output against 2-3-stable-mrh. That strategy failed ;)