Monitor connection count in production

The idea here is that, according to our reasoning, we should never have more than 3 checked out DB connections at once per process: two threads running in a Puma worker plus one pubsub thread. This connection monitor logs an error whenever more than 3 connections are checked out, including the call stacks when each connection was checked out so that we can hopefully diagnose which parts of the app are checking out additional connections.

A corresponding Papertrail search will need to be set up to notify us when we have > 3 connections.