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 I do not want to tune the API for upstream acceptability at this point. I'm targeting a *minimal* working version we can validate in production soon. I'd dislike to add any unused semantics to the `MemoryScope` proactively.