code: func NewCrawlConfig() *Config {
  	config := NewStandardConfig()
  	config.NodeExpriedAfter = 0
  	config.KBucketExpiredAfter = 0
  	config.CheckKBucketPeriod = time.Second * 5
  	config.KBucketSize = math.MaxInt32
  	config.Mode = CrawlMode
  	config.RefreshNodeNum = 256
  
  	return config
  }
docs: // NewCrawlConfig returns a config in crawling mode.