推荐第四种：https://github.com/shawntime/shawn-rwdb
4种不方的读写分离实现方法
http://blog.csdn.net/lixiucheng005/article/details/17391857
http://blog.csdn.net/xtj332/article/details/43953699
http://www.jianshu.com/p/66e5cf1024ca
