API: Add DB connection check to health check?

[QuantumDancer]

Right now the health check (/health_check) just returns a 200 OK. This shows that Auditor is running but does not test if the DB connection works. As Auditor itself is stateless we store all the state in the DB. Therefore, we should think about if incorporating a DB connection check to the health check, because without it all requests will fail. The downside of this is that the health check becomes more expensive to perform and that more load is put on the DB.