Option for a cache-busting cert-url

Consider an option to include `?YYYYMMDD` in the cert-url so that it busts any caches that don't respect the cache-control header.

The issue names no files or tests. Start by locating the cert-url handling in the Go code and examining how cache-control is applied; done means an option can add a ?YYYYMMDD suffix to cert-url for cache busting, with behavior covered by the relevant tests if they exist.

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