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.
Yeah. This is our fork. And we'll also have the need in future to use mutant to check our specs. I can move the dependency addition in a separate commit we can safely ignore when pushing this one upstream. I'll do this after addressing all other code flags to not screw up github comment attachments while fixing other flags.