修改json 为 gson
#92 为Name Server增加日志
Merge branch 'master' of gitlab.alibaba-inc.com:shijia.wxr/rocketmq
fixed #96 通信层自定义字段有空值校验，PullMessageRequestHeader有字段可能为空
    @CFNullable
    private String subscription;
Merge https://github.com/alibaba/RocketMQ