support log rotate about log15 HOT 3 OPEN

//log rotate: 1.change file handler by date. 2.remove expired log file
func logRotate(expiredDays int, prefixFile string, f log.Format, lvl log.Lvl, logger log.Logger) {
	for {
		lastFileHandler := setLogFileHandler(prefixFile, f, lvl, logger)
		now := time.Now()
		next := time.Date(now.Year(), now.Month(), now.Day() + 1, 0, 0, 0, 0, now.Location())
		t := time.NewTimer(next.Sub(now))
		<-t.C
		// 1.change log file to date
		temp := setLogFileHandler(prefixFile, f, lvl, L)
		//lastFileHandler.Close()  //Close file. the struct of closingHandler not exported!
		reflect.ValueOf(lastFileHandler).MethodByName("Close").Call([]reflect.Value{})
		lastFileHandler = temp
		// TODO 2.remove expired log file
		//
	}
}

//get log file name
func getLogFileName(prefixFile string, t time.Time) string {
	return fmt.Sprintf("%v-%04d-%02d-%02d.log", prefixFile, t.Year(), t.Month(), t.Day())
}

//set log handler, and return file handler.
func setLogFileHandler(prefixFile string, f log.Format, lvl log.Lvl, logger log.Logger) log.Handler {
	curFilename := getLogFileName(prefixFile, time.Now())
	fileHandler := log.Must.FileHandler(curFilename, f)
	logger.SetHandler(log.MultiHandler(
		log.LvlFilterHandler(
			lvl,
			log.CallerFileHandler(log.StdoutHandler)),
		log.LvlFilterHandler(
			lvl,
			fileHandler)))
	return fileHandler
}