ServiceBackend interface (#369)

* Add ServiceBackend interface

This adds a ServiceBackend interface that is shared between etcd/etcd3
(later) and kubernetes, leading to a massive reduction in code. When
returning the specific records from their backend.

Fixes #273

middleware/etcd/debug.go

@@ -1,12 +1,6 @@
 package etcd
 
-import (
-	"strings"
-
-	"github.com/miekg/coredns/middleware/etcd/msg"
-
-	"github.com/miekg/dns"
-)
+import "strings"
 
 const debugName = "o-o.debug."
 
@@ -24,34 +18,3 @@ func isDebug(name string) string {
 	}
 	return name[len(debugName):]
 }
-
-// servicesToTxt puts debug in TXT RRs.
-func servicesToTxt(debug []msg.Service) []dns.RR {
-	if debug == nil {
-		return nil
-	}
-
-	rr := make([]dns.RR, len(debug))
-	for i, d := range debug {
-		rr[i] = d.RR()
-	}
-	return rr
-}
-
-func errorToTxt(err error) dns.RR {
-	if err == nil {
-		return nil
-	}
-	msg := err.Error()
-	if len(msg) > 255 {
-		msg = msg[:255]
-	}
-	t := new(dns.TXT)
-	t.Hdr.Class = dns.ClassCHAOS
-	t.Hdr.Ttl = 0
-	t.Hdr.Rrtype = dns.TypeTXT
-	t.Hdr.Name = "."
-
-	t.Txt = []string{msg}
-	return t
-}