code: func clientSessionCacheKey(serverAddr net.Addr, config *Config) string {
  	if len(config.ServerName) > 0 {
  		return config.ServerName
  	}
  	return serverAddr.String()
  }
docs: // clientSessionCacheKey returns a key used to cache sessionTickets that could
  // be used to resume previously negotiated TLS sessions with a server.