middleware/proxy: implement Exchanger (#480)

By defining and using an proxy.Exchanger interface we make the proxy
more generic and we can then fold back httproxy into proxy.

This overrides #463 and #473 and should make futures extensions rather
trivial

* Add docs that talk about `protocol` and how to set it.
* middleware/proxy: rename New to NewLookup
  It's used as a Lookup mechanism not as a completely new proxy,
  reflect that in the name.
* Set maxfails to 3 by default when looking up names.

Most of the changes have been copied
from https://github.com/johnbelamaric/coredns/pull/1/files

1 files changed, 3 insertions, 4 deletions

diff --git a/test/etcd_test.go b/test/etcd_test.go
index f627e3872..145b83a48 100644
--- a/test/etcd_test.go
+++ b/test/etcd_test.go
@@ -66,15 +66,14 @@ func TestEtcdStubAndProxyLookup(t *testing.T) {
 		defer delete(ctx, t, etc, serv.Key)
 	}
 
-	p := proxy.New([]string{udp}) // use udp port from the server
+	p := proxy.NewLookup([]string{udp}) // use udp port from the server
 	state := request.Request{W: &test.ResponseWriter{}, Req: new(dns.Msg)}
 	resp, err := p.Lookup(state, "example.com.", dns.TypeA)
 	if err != nil {
-		t.Error("Expected to receive reply, but didn't")
-		return
+		t.Fatalf("Expected to receive reply, but didn't", err)
 	}
 	if len(resp.Answer) == 0 {
-		t.Error("Expected to at least one RR in the answer section, got none")
+		t.Fatalf("Expected to at least one RR in the answer section, got none")
 	}
 	if resp.Answer[0].Header().Rrtype != dns.TypeA {
 		t.Errorf("Expected RR to A, got: %d", resp.Answer[0].Header().Rrtype)