Add fudge factor in number of impressions requested from adzerk.
For initial release use weights rather than CPM goals.

Setting adzerk Flights to serve a percentage goal will be similar
to our existing weighted delivery system. Each flight will get a
number of impressions proportional to its daily bid.
starting on refunding completed campaigns
update PromotionLog on update_adzerk
deactivate_campaign only update adzerk if needed
let r2.lib.promote handle billable calculations
update adzerk on promote.charge_pending