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_test.go

@@ -0,0 +1,66 @@
+package out
+
+import (
+	"net"
+	"testing"
+)
+
+func sendOneTCP(tcp *TCP) error {
+	if _, err := tcp.Write([]byte("frame")); err != nil {
+		return err
+	}
+	if err := tcp.Flush(); err != nil {
+		return err
+	}
+	return nil
+}
+func TestTCP(t *testing.T) {
+	tcp := NewTCP("localhost:14000")
+
+	if err := sendOneTCP(tcp); err == nil {
+		t.Fatal("Not listening but no error.")
+		return
+	}
+
+	l, err := net.Listen("tcp", "localhost:14000")
+	if err != nil {
+		t.Fatal(err)
+		return
+	}
+
+	wait := make(chan bool)
+	go func() {
+		acceptOne(t, l)
+		wait <- true
+	}()
+
+	if err := sendOneTCP(tcp); err != nil {
+		t.Fatalf("send one: %s", err)
+		return
+	}
+
+	<-wait
+
+	// TODO: When the server isn't responding according to the framestream protocol
+	// the thread is blocked.
+	/*
+		if err := sendOneTCP(tcp); err == nil {
+			panic("must fail")
+		}
+	*/
+
+	go func() {
+		acceptOne(t, l)
+		wait <- true
+	}()
+
+	if err := sendOneTCP(tcp); err != nil {
+		t.Fatalf("send one: %s", err)
+		return
+	}
+
+	<-wait
+	if err := l.Close(); err != nil {
+		t.Error(err)
+	}
+}