feat: api can return json fomart now

feat: pull on subscribe event use InvitePublish to instead *Stream
fix: pull remote stream publish cause bugs
fix: Concurrency MarshalJSON Tracks

desc:
- API 现在可以返回{"data":xx,"code":0,"msg":"ok"}格式
- 按需拉流的事件的类型从*Stream修改为InvitePublish
- 远程拉流频繁重连后导致多路远程流同时写入同一个track
- 在并发序列化Tracks时导致错误，通过加lock解决

puller.go

@@ -0,0 +1,85 @@
+package engine
+
+import (
+	"io"
+	"time"
+
+	"go.uber.org/zap"
+	"m7s.live/engine/v4/config"
+)
+
+var zshutdown = zap.String("reason", "shutdown")
+var znomorereconnect = zap.String("reason", "no more reconnect")
+
+type IPuller interface {
+	IPublisher
+	Connect() error
+	Disconnect()
+	Pull() error
+	Reconnect() bool
+	init(streamPath string, url string, conf *config.Pull)
+	startPull(IPuller)
+}
+
+// 用于远程拉流的发布者
+type Puller struct {
+	ClientIO[config.Pull]
+}
+
+// 是否需要重连
+func (pub *Puller) Reconnect() (ok bool) {
+	ok = pub.Config.RePull == -1 || pub.ReConnectCount <= pub.Config.RePull
+	pub.ReConnectCount++
+	return
+}
+
+func (pub *Puller) startPull(puller IPuller) {
+	badPuller := true
+	var stream *Stream
+	var err error
+	Pullers.Store(puller, pub.RemoteURL)
+	defer func() {
+		Pullers.Delete(puller)
+		puller.Disconnect()
+		if stream != nil {
+			stream.Close()
+		}
+	}()
+	puber := puller.GetPublisher()
+	originContext := puber.Context // 保存原始的Context
+	for puller.Info("start pull"); puller.Reconnect(); puller.Warn("restart pull") {
+		if err = puller.Connect(); err != nil {
+			if err == io.EOF {
+				puller.Info("pull complete")
+				return
+			}
+			puller.Error("pull connect", zap.Error(err))
+			if badPuller {
+				return
+			}
+			time.Sleep(time.Second * 5)
+		} else {
+			puber.Context = originContext // 每次重连都需要恢复原始的Context
+			if err = puller.Publish(pub.StreamPath, puller); err != nil {
+				puller.Error("pull publish", zap.Error(err))
+				return
+			}
+			s := puber.Stream
+			if stream != s && stream != nil { // 这段代码说明老流已经中断，创建了新流，需要把track置空，从而避免复用
+				puber.AudioTrack = nil
+				puber.VideoTrack = nil
+			}
+			stream = s
+			badPuller = false
+			if err = puller.Pull(); err != nil && !puller.IsShutdown() {
+				puller.Error("pull interrupt", zap.Error(err))
+			}
+		}
+		if puller.IsShutdown() {
+			puller.Info("stop pull", zshutdown)
+			return
+		}
+		puller.Disconnect()
+	}
+	puller.Warn("stop pull", znomorereconnect)
+}