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.
Rather than this being a controller filter I think it should become part of the order state machine transitions.  We need this to apply for all api, backend, and frontend order cases.