logging.go
func (logger *Logger) SetLevel(level Level) {
	logger.lock.Lock()
	logger.Level = level
	logger.flushBuffer()
	logger.lock.Unlock()
}
func (logger *Logger) SetLevel(level Level) {
	logger.lock.Lock()
	logger.Level = level
	logger.flushBuffer()
	logger.lock.Unlock()
}
// SetLevel sets lower level that current logger will process.