QueuePool limit of size overflow when frequently login and out

[yanqingjing]

I followed the document example of flask security with sqlAlchemy from: basic-sqlalchemy-application-with-session

When I was testing the login function, after frequent login and logout（more than 20 time in one minute）, the application will crash down and raise exception:

sqlalchemy.exc.TimeoutError: QueuePool limit of size 10 overflow 10 reached, connection timed out, timeout 30 (Background on this error at: http://sqlalche.me/e/13/3o7r)