mongo group by mapreduce

http://jackyrong.iteye.com/blog/1408548
http://www.cnblogs.com/loogn/archive/2012/02/09/2344054.html
http://www.tuicool.com/articles/FVB7jyI
http://www.iteye.com/news/28013
http://www.linuxidc.com/Linux/2015-09/123671.htm 16M
https://docs.mongodb.com/manual/reference/operator/aggregation/group/ limit限制
https://veryyoung.me/blog/2015/10/18/mongodb-aggregation-16m-limit.html 16M 好

http://www.infoq.com/cn/articles/implementing-aggregation-functions-in-mongodb