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 suppose you could also write this as:

```ruby
variant = order.line_items.detect { |line_item| line_item.variant_id.equal?(variant_id) }.try(:variant) ||
  Spree::Variant.find(variant_id)
```