Negative revalidating and DNS error responses

Figured Id open this just so we can have a discussion during one of our calls

We noticed an issue where if you get back a dns error response for an upstream lookup that no matter what your `negative_revalidating_enabled` setting is, ATS will reply with a stale response. Since we run with neg_reval disabled this came as sort of surprise during this scenario so looking it seems like it only comes in to play after an upstream response of some sort has been received (or a connection error).

So just bringing it up here to see if maybe we wanted to extend this to also include DNS error responses on the upstream lookup as well?