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.
See my comment [here](https://github.com/MountainRoseHerbs/spree/pull/12/files#r18407830) and substitute `Spree::LineItem` with `Spree::Shipment`.