* middleware/httproxy: implement debug queries
Not too useful at the moment, but o-o.debug queries are supported
and return the Comment from dns.google.com.
Note that this is not always set.
* improve documentation
* Testing cleanups
This PR adds a middleware that talks to dns.google.com over HTTPS,
meaning all your DNS traffic is encrypted when traversing your ISP and
the internet.
The `dns.google.com` address is re-resolved every 30 seconds.
