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/msg_test.go

@@ -14,7 +14,7 @@ import (
 
 func testRequest(t *testing.T, expected Data, r request.Request) {
 	d := Data{}
-	if err := d.FromRequest(r); err != nil {
+	if err := d.RemoteAddr(r.W.RemoteAddr()); err != nil {
 		t.Fail()
 		return
 	}