perfrun fetches the latest N successful builds from the dashboard and
runs their go1 benchmarks, saving logs on disk
This is the start of a daemon that will automatically run the benchmarks
for every successful build and archive the logs to Cloud Storage.
Change-Id: I5dfd715788919abaf22f71a206f2682eceeaadcc
Reviewed-on: https://go-review.googlesource.com/29399
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>