The `__heartbeat__` route should check db health

[rfk]

In production right now, we've got a node that is throwing 503s for user traffic, but happily reporting 200 Everything Is Awesome!! 👍 😁 to our monitoring scripts via the __heartbeat__ route. It would be useful if the heartbeat could do a basic healthcheck on the db.

(I recall an old issue about this, but I couldn't find it, so filing a new one)