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/out/tcp.go

@@ -0,0 +1,59 @@
+package out
+
+import (
+	"net"
+	"time"
+
+	fs "github.com/farsightsec/golang-framestream"
+)
+
+// TCP is a Frame Streams encoder over TCP.
+type TCP struct {
+	address string
+	frames  [][]byte
+}
+
+// NewTCP returns a TCP writer.
+func NewTCP(address string) *TCP {
+	s := &TCP{address: address}
+	s.frames = make([][]byte, 0, 13) // 13 messages buffer
+	return s
+}
+
+// Write a single Frame Streams frame.
+func (s *TCP) Write(frame []byte) (n int, err error) {
+	s.frames = append(s.frames, frame)
+	if len(s.frames) == cap(s.frames) {
+		return len(frame), s.Flush()
+	}
+	return len(frame), nil
+}
+
+// Flush the remaining frames.
+func (s *TCP) Flush() error {
+	defer func() {
+		s.frames = s.frames[0:]
+	}()
+	c, err := net.DialTimeout("tcp", s.address, time.Second)
+	if err != nil {
+		return err
+	}
+	enc, err := fs.NewEncoder(c, &fs.EncoderOptions{
+		ContentType:   []byte("protobuf:dnstap.Dnstap"),
+		Bidirectional: true,
+	})
+	if err != nil {
+		return err
+	}
+	for _, frame := range s.frames {
+		if _, err = enc.Write(frame); err != nil {
+			return err
+		}
+	}
+	return enc.Flush()
+}
+
+// Close is an alias to Flush to satisfy io.WriteCloser similarly to type Socket.
+func (s *TCP) Close() error {
+	return s.Flush()
+}