slightly improved http error status codes and db health check

[chunningham]

Description

DB connection failing to be acquired from the connection pool currently results in a 401 unauthorised status code, when it should probably be a 500 or 503 status. This PR sets it to give a 503 status in this case. not necessarily a bug fix but will be more informative in future. Additionally, this pr sets the max number of connections to 100 (previously defaulted to 10) and adds a db status check to the /healthz endpoint.

Type

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Diligence Checklist

(Please delete options that are not relevant)

• This change requires a documentation update
• I have included unit tests
• I have updated and/or included new integration tests
• I have updated and/or included new end-to-end tests
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[chunningham]

@sbihel the default max connections was 10, I've set it to 100, perhaps that's too high but I'm not sure how to estimate an appropriate value for a given deployment. I could also add it as a config option

[sbihel]

Yeah potentially making it configurable would be nice. I think 100 is fine for now