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/backend.go

@@ -0,0 +1,29 @@
+package middleware
+
+import (
+	"github.com/miekg/coredns/middleware/etcd/msg"
+	"github.com/miekg/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// ServiceBackend defines a (dynamic) backend that returns a slice of service definitions.
+type ServiceBackend interface {
+	// Services communitates with the backend to retrieve the service defintion. Exact indicates
+	// on exact much are that we are allowed to recurs.
+	Services(state request.Request, exact bool, opt Options) ([]msg.Service, []msg.Service, error)
+
+	// Lookup is used to find records else where.
+	Lookup(state request.Request, name string, typ uint16) (*dns.Msg, error)
+
+	// IsNameError return true if err indicated a record not found condition
+	IsNameError(err error) bool
+
+	// Debug returns a string used when returning debug services.
+	Debug() string
+}
+
+// Options are extra options that can be specified for a lookup.
+type Options struct {
+	Debug string // This is a debug query. A query prefixed with debug.o-o
+}