On finishing up a scraper run remove logs from old scraper runs

[mlandauer]

We currently have a rake task which clears out old logs but only running this occasionally is problematic. Doing it in an emergency is not so great. For instance just running it yesterday (and doing a table optimize) reduced disk usage by 80GB, not an amount to sniff at.

So it would be better if we were constantly deleting the old scraper logs. Really there is very little use in keeping the old ones around I think.