Adds the dnstap I/O routines and should fix some issues (#1083)

* adds the dnstap I/O thread and should fix a lot of mistakes

* docs

* -race test

* oops

* docs

plugin/dnstap/dnstapio/io_test.go

@@ -0,0 +1,71 @@
+package dnstapio
+
+import (
+	"bytes"
+	"sync"
+	"testing"
+	"time"
+
+	tap "github.com/dnstap/golang-dnstap"
+)
+
+type buf struct {
+	*bytes.Buffer
+	cost time.Duration
+}
+
+func (b buf) Write(frame []byte) (int, error) {
+	time.Sleep(b.cost)
+	return b.Buffer.Write(frame)
+}
+
+func (b buf) Close() error {
+	return nil
+}
+
+func TestRace(t *testing.T) {
+	b := buf{&bytes.Buffer{}, 100 * time.Millisecond}
+	dio := New(b)
+	wg := &sync.WaitGroup{}
+	wg.Add(10)
+	for i := 0; i < 10; i++ {
+		timeout := time.After(time.Second)
+		go func() {
+			for {
+				select {
+				case <-timeout:
+					wg.Done()
+					return
+				default:
+					time.Sleep(50 * time.Millisecond)
+					dio.Dnstap(tap.Dnstap{})
+				}
+			}
+		}()
+	}
+	wg.Wait()
+}
+
+func TestClose(t *testing.T) {
+	done := make(chan bool)
+	var dio *DnstapIO
+	go func() {
+		b := buf{&bytes.Buffer{}, 0}
+		dio = New(b)
+		dio.Close()
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("Not closing.")
+	}
+	func() {
+		defer func() {
+			if err := recover(); err == nil {
+				t.Fatal("Send on closed channel.")
+			}
+		}()
+		dio.Dnstap(tap.Dnstap{})
+	}()
+}