When promotions conflict due to stackability use the promotion that produces the largest discount amount

[zenweasel]

When one or more promotions are to be applied but cannot because of their stackability, the system needs to calculate which will produce the largest discount amount. I realize that this may produce a performance issue if we do this every time so we need to figure out how to optimize this calculation so that it only needs to be done the minimum amount of times.

Note that I think this only applies to "Offers", not to coupons (because the customer caused the offer to happen) or for shipping discounts since we won't know what the amount is until checkout.