middleware/proxy: dnstap (#786)

* experimental dnstap support into proxy

* proxy reports dnstap errors

* refactoring

* add a message builder for less dnstap code

* msg lint

* context

* proxy by DNS: dnstap comments

* TapBuilder

* resolves conflict

* dnstap into ServeDNS

* testing

* more tests

* `go lint`

* doc update

middleware/proxy/google.go

@@ -112,6 +112,10 @@ func (g *google) exchangeJSON(addr, json string) ([]byte, error) {
 	return buf, nil
 }
 
+func (g *google) Transport() string {
+	return "tcp"
+}
+
 func (g *google) Protocol() string { return "https_google" }
 
 func (g *google) OnShutdown(p *Proxy) error {