mysql建立的連接要及時刪除，不然連接池資源耗盡
相關文章參考：
http://blog.csdn.net/robinson1988/article/details/4713294
http://blog.itpub.net/9399028/viewspace-682415/
http://blog.csdn.net/u013673976/article/details/45939297
http://blog.itpub.net/23718752/viewspace-1279972/
http://docs.sqlalchemy.org/en/latest/core/pooling.html