Make max number of idle connections configurable.

[blkt]

Summary

This change allows setting the maximum number of idle connections via configuration file. This is useful since local testing showed that Minder eventer spends roughly ~25% of its time opening connections to Postgres.

Change Type

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Checked via pprof and debugger.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 52.843% (+0.002%) from 52.841%
when pulling 909d10c on enh/make-idle-connections-configurable
into 9054b74 on main.