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/dnstap/msg/wrapper.go

@@ -15,6 +15,7 @@ func wrap(m *lib.Message) *lib.Dnstap {
 	}
 }
 
+// Marshal encodes the message to a binary dnstap payload.
 func Marshal(m *lib.Message) (data []byte, err error) {
 	data, err = proto.Marshal(wrap(m))
 	if err != nil {