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 ended up with providing a `MemoryScope#find` that does not do any coerce and coercing via `params[:id].to_i(10)` on the call side as this is most close to original semantics in terms of exceptions being raised and finder behavior.