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/taprw/writer.go

@@ -0,0 +1,73 @@
+// Package taprw takes a query and intercepts the response.
+// It will log both after the response is written.
+package taprw
+
+import (
+	"fmt"
+
+	"github.com/coredns/coredns/plugin/dnstap/msg"
+
+	tap "github.com/dnstap/golang-dnstap"
+	"github.com/miekg/dns"
+)
+
+// Tapper is what ResponseWriter needs to log to dnstap.
+type Tapper interface {
+	TapMessage(m *tap.Message) error
+	TapBuilder() msg.Builder
+}
+
+// ResponseWriter captures the client response and logs the query to dnstap.
+// Single request use.
+type ResponseWriter struct {
+	queryEpoch uint64
+	Query      *dns.Msg
+	dns.ResponseWriter
+	Tapper
+	err error
+}
+
+// DnstapError check if a dnstap error occurred during Write and returns it.
+func (w ResponseWriter) DnstapError() error {
+	return w.err
+}
+
+// QueryEpoch sets the query epoch as reported by dnstap.
+func (w *ResponseWriter) QueryEpoch() {
+	w.queryEpoch = msg.Epoch()
+}
+
+// WriteMsg writes back the response to the client and THEN works on logging the request
+// and response to dnstap.
+// Dnstap errors are to be checked by DnstapError.
+func (w *ResponseWriter) WriteMsg(resp *dns.Msg) (writeErr error) {
+	writeErr = w.ResponseWriter.WriteMsg(resp)
+	writeEpoch := msg.Epoch()
+
+	b := w.TapBuilder()
+	b.TimeSec = w.queryEpoch
+	if err := func() (err error) {
+		err = b.AddrMsg(w.ResponseWriter.RemoteAddr(), w.Query)
+		if err != nil {
+			return
+		}
+		return w.TapMessage(b.ToClientQuery())
+	}(); err != nil {
+		w.err = fmt.Errorf("client query: %s", err)
+		// don't forget to call DnstapError later
+	}
+
+	if writeErr == nil {
+		if err := func() (err error) {
+			b.TimeSec = writeEpoch
+			if err = b.Msg(resp); err != nil {
+				return
+			}
+			return w.TapMessage(b.ToClientResponse())
+		}(); err != nil {
+			w.err = fmt.Errorf("client response: %s", err)
+		}
+	}
+
+	return
+}