use only completed promotion usages when checking usage limit

It should only count completed orders against the usage_limit.  Counting incomplete orders as using a coupon doesn't make sense.  We could also probably take the promotable argument off of the usage_limit_exceeded? method signature but I wasn't sure that was a good idea.
this and the above returns would break encapsulation. The function of these methods is to advance the order, adding conditional returns here adds a separate responsibility that is difficult to understand.