add heartbeat echo from server to client

src/frp/models/client/config.go

@@ -15,6 +15,7 @@ var (
 	LogLevel          string = "warn"
 	LogWay            string = "file"
 	HeartBeatInterval int64  = 5
+	HeartBeatTimeout  int64  = 30
 )
 
 var ProxyClients map[string]*ProxyClient = make(map[string]*ProxyClient)

src/frp/models/consts/consts.go

@@ -11,3 +11,13 @@ const (
 	CtlConn = iota
 	WorkConn
 )
+
+// msg from client to server
+const (
+	CSHeartBeatReq = 1
+)
+
+// msg from server to client
+const (
+	SCHeartBeatRes = 100
+)