Stick with object given in Adjustment#update!

Help us fixes a couple more specs where the order reference was being
lost
Give an Order to `compute_amount` where needed

For line item adjustments order was being reloaded every time one had to
compute calculate the adjustment amount

Theres probably a better way to build this `compute_amount` interface,
code looks dirty, but at least we have a better direction now on how
to avoid Order Loads when we dont really have to.