middleware/httpproxy: Add (#439)

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.

middleware/httpproxy/tls.go

@@ -0,0 +1,32 @@
+package httpproxy
+
+import (
+	"crypto/tls"
+	"net/http"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// Exchanger is an interface that specifies a type implementing a DNS resolver that
+// uses a HTTPS server.
+type Exchanger interface {
+	Exchange(*dns.Msg) (*dns.Msg, error)
+
+	SetUpstream(*simpleUpstream) error
+	OnStartup() error
+	OnShutdown() error
+}
+
+func newClient(sni string) *http.Client {
+	tls := &tls.Config{ServerName: sni}
+
+	c := &http.Client{
+		Timeout:   time.Second * timeOut,
+		Transport: &http.Transport{TLSClientConfig: tls},
+	}
+
+	return c
+}
+
+const timeOut = 5