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.
I prefer to use `block_given?` since it is optimized inside most implementations and its the more strict version of a truthiness check on an lvar. We tend to always use the most strict / specialized operation available.