Reduce unneeded pr checking

While looking at our logs I noticed we bombarded jenkins when we got our first PR for the gh-pages branch.
This PR need not be validated, and we won't succeed in starting a job, so we keep asking jenkins where it is.

While I was at it, I also added some logging to see how much we could save by looking at the last-updated timestamp of a PR and only acting when it's later than last time we checked.

We could increase our checking frequency if we make fewer API calls each iteration.