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 Will `Time.at(0)` be affected by ENV variable setup such as `TZ` that might happen after the constant got evaluated on load time? - If not I totally support this change.