Review connection pooling code

[jameshiebert]

There has been some skepticism that our connection pooling code is properly releasing the connections. We've had a few instances in production where the number of used connections has slowly crept up.

We should take the time in the future to review the connection pooling, ensure that it is using the most recent version of SQLAlchemy, following best practices, and ensure that the pooling code is actually covered by our tests.