Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/dnstap/msg/wrapper.go

@@ -0,0 +1,26 @@
+package msg
+
+import (
+	"fmt"
+
+	lib "github.com/dnstap/golang-dnstap"
+	"github.com/golang/protobuf/proto"
+)
+
+func wrap(m *lib.Message) *lib.Dnstap {
+	t := lib.Dnstap_MESSAGE
+	return &lib.Dnstap{
+		Type:    &t,
+		Message: m,
+	}
+}
+
+// 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 {
+		err = fmt.Errorf("proto: %s", err)
+		return
+	}
+	return
+}