code: public <T> ServerBuilder childChannelOption(ChannelOption<T> option, T value) {
          requireNonNull(option, "option");
          checkArgument(!PROHIBITED_SOCKET_OPTIONS.contains(option),
                        "prohibited socket option: %s", option);
  
          option.validate(value);
          childChannelOptions.put(option, value);
          return this;
      }
docs: Sets the {@link ChannelOption} of sockets accepted by {@link Server}.
  Note that the previously added option will be overridden if the same option is set again.
  
  <pre>{@code
  ServerBuilder sb = new ServerBuilder();
  sb.childChannelOption(ChannelOption.SO_REUSEADDR, true)
  .childChannelOption(ChannelOption.SO_KEEPALIVE, true);
  }</pre>