feat(server): multi-region aware liveness and readiness checks

[iainsproat]

Description & motivation

Determines if all databases are up and have sufficient free connections in the knex connection pool.
This informs the liveness and readiness checks.

Changes:

• Separates the various components of the liveness and readiness checks into different files for readability
• Adding a new region will add a new metric to the liveness and readiness endpoints

To-do before merge:

• update the calculation of free connections when a region is added without incurring a restart of the app.
• check if ServerInfo remains a valid test of whether the database connection is working

Screenshots:

Validation of changes:

Checklist:

• My pull request follows the guidelines in the Contributing guide?
• My pull request does not duplicate any other open Pull Requests for the same update/change?
• My commits are related to the pull request and do not amend unrelated code or documentation.
• My code follows a similar style to existing code.
• I have added appropriate tests.
• I have updated or added relevant documentation.

References

[linear]

WEB-2142 Multi region DB connection health

[gjedlicska]

@iainsproat tests are failing on this one