Speed up rename_adjustment_fields migration

I went to perform this migration on our production db while our store was in maintenance mode and realized it was going to take many hours -- it was loading some million or so Order objects, one at a time.

The first change is that we go from loading all Adjustments and asking whether _each_  one's `adjustable` is an Order to just scoping the Adjustment call to only ask for the ones that are. That saved us about one million `if` statements.

The second change is to simply copy the `adjustable_id` directly over to `order_id` without ever fetching the Order record -- who needs it? This saved us about one million ActiveRecord queries.

Happy to pass the fruits along to others on the 2.1--2.2 upgrade path!