2020-08-27 Alibaba Cloud.

* version: 2.4.11
* IMPROVE: add ut_test script.
* IMPORVE: in full sync stage: replace mgo driver with mongo-go-driver in
reading.
* BUGFIX: if can not run incr sync directly, return error when sync_mode
== incr.
* IMPROVE: docSync crash directly when meets error in full sync.
* BUGFIX: do not change sync_mode whn tunnel != direct. #424
* IMPROVE: sort data before return in full sync stage.

1. 添加单测的脚本
2. 全量阶段读取的驱动由mgo改为mongo-go-driver
3. 修复bug：sync_mode==incr，如无法直接进入增量同步，则直接退出
4. 全量同步阶段，报错则直接crash退出，不等待全量同步完毕再crash
5. 修复bug：如果tunnel不为direct，sync_mode不会进行自动判断转换。#424
6. 全量阶段返回的数据进行排序