sqlalchemy: TimeoutError: QueuePool limit of size 5 overflow 10 reached, connection timed out, timeout 30

mysql建立的连接要及时删除，不然连接池资源耗尽

相关文章参考：

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