An (artificial) originator for credit adjustments

This lets us automatically recalculate the Store Credit adjustment when
the order changes.
Force the adjustments to recalculate in order

1. Shipping
2. Promotions
3. Tax
4. Credits
Log whenever a credit is used.
Add credit expirations
Don't create a credit adjustment when credits == 0
Always check store credits when moving to payment

Previously, the store credits adjustment was only created the first time
the order transitioned to the "payment" state, and only if there was a
balance. Now the adjustment will be correctly created even if we
previously went to the payment state with a no-charge cart.