fix the memory leak on the wait API

The pollForIndex method starts a goroutine to check the latest index
periodically. But when the HTTP request finishs, the loop is still there
and the goroutine never exits.

This leads to a memory leak and goroutine leak after each request to the
"/wait/:index" API.

The problem can be reproduced by ```ab -c 10 -t 30 http://127.0.0.1:8086/wait/1```.
After several times of benchmark you'll find that the memory usage grows markedly.

Signed-off-by: Shijiang Wei <mountkin@gmail.com>

httpd/handler.go

@@ -308,36 +308,68 @@ func (h *Handler) serveWait(w http.ResponseWriter, r *http.Request) {
 	} else {
 		d = time.Duration(timeout) * time.Millisecond
 	}
-	err := h.pollForIndex(index, d)
-	if err != nil {
-		w.WriteHeader(http.StatusRequestTimeout)
-		return
+	poller := &indexPoller{
+		h:    h,
+		quit: make(chan bool),
+	}
+	if notify, ok := w.(http.CloseNotifier); ok {
+		go func(poller *indexPoller) {
+			<-notify.CloseNotify()
+			poller.Quit()
+		}(poller)
+	}
+
+	err := poller.PollForIndex(index, d)
+	switch err {
+	case errPollTimedOut:
+		w.WriteHeader(http.StatusRequestTimeout)
+	case nil:
+		w.Write([]byte(fmt.Sprintf("%d", h.server.Index())))
 	}
-	w.Write([]byte(fmt.Sprintf("%d", h.server.Index())))
 }
 
-// pollForIndex will poll until either the index is met or it times out
-// timeout is in milliseconds
-func (h *Handler) pollForIndex(index uint64, timeout time.Duration) error {
-	done := make(chan struct{})
+type indexPoller struct {
+	h    *Handler
+	quit chan bool
+}
 
+var (
+	errPollTimedOut = errors.New("timed out")
+	errAborted      = errors.New("aborted")
+)
+
+func (p *indexPoller) PollForIndex(index uint64, timeout time.Duration) error {
+	aborted := make(chan bool)
+	done := make(chan bool)
 	go func() {
 		for {
-			if h.server.Index() >= index {
-				done <- struct{}{}
+			select {
+			case <-p.quit:
+				aborted <- true
+				return
+			default:
+				if p.h.server.Index() >= index {
+					done <- true
+					return
+				}
+				time.Sleep(10 * time.Millisecond)
 			}
-			time.Sleep(10 * time.Millisecond)
 		}
 	}()
 
-	for {
-		select {
-		case <-done:
-			return nil
-		case <-time.After(timeout):
-			return fmt.Errorf("timed out")
-		}
+	select {
+	case <-time.After(timeout):
+		return errPollTimedOut
+	case <-aborted:
+		return errAborted
+	case <-done:
+		return nil
 	}
+	return nil
+}
+
+func (p *indexPoller) Quit() {
+	p.quit <- true
 }
 
 // serveDataNodes returns a list of all data nodes in the cluster.