Use AltSvc frames to discover more DoH endpoints

[sebdeckers]

A DoH server may send AltSvc frames to Dohnut. These should be treated as alternative connections for load balancing purposes. I.e. discover a lower latency edge server.

With GeoDNS it's not always clear which edge server is lower latency. Network distance !== map distance. For long-lived DoH connections this may provide a self-optimising solution.