* middleware/proxy: absorb httpproxy
Move the httproxy into proxy. This adds and Exchanger interface which
is used to exchange the messages with the upstream.
The https_google upstream will re-resolve itself and update the upstream
hosts used every 300s.
* Remove and add TODO
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.