HostDB's "stale while revalidate" doesn't span failed DNS lookups

Right now if you set stale_while_revalidate hostdb will return the stale record while waiting on the DNS lookup. In the event that you have the DNS timeout set low (lets say 1 second) and the DNS server times out (lets say due to an outage of your resolver) then ATS will start replacing all of your stale entries (which were working) with error timeout entries (which are empty)-- which is bad.

The stale while revalidate configuration should serve stale records until a valid response from DNS comes back-- the patch here should be relatively simple, just a bit of a nuisance to test since DNS requires port 53