feat: support max running tasks config (#112)

pkg/base/constants.go

@@ -3,9 +3,10 @@ package base
 type Status string
 
 const (
-	DownloadStatusReady   Status = "ready"
+	DownloadStatusReady   Status = "ready" // task create but not start
 	DownloadStatusRunning Status = "running"
 	DownloadStatusPause   Status = "pause"
+	DownloadStatusWait    Status = "wait" // task is wait for running
 	DownloadStatusError   Status = "error"
 	DownloadStatusDone    Status = "done"
 )