Adzerk tweaks

2 changes to speed up ```POST_request_promo``` responses:
* Don't lookup ads before making the request to adzerk. We already checked whether there were ads when building the Frontpage or Subreddit listing, so there's no need to check again.
* Only request 1 ad from adzerk instead of 3 (```g. az_selfserve_num_request```). I talked to James from Adzerk and he said the response time is approximately linear with the number of ads requested, so this should be a big improvement. We'd been requesting 3 ads to have extras in case some were skipped in the builder, but it's probably ok to just not show anything in that case. In the future we can get fancy by making a second request to adzerk for multiple ads if the ad gets skipped.