chore: Refactor internal/database/dbconn

Created by: efritz

This is me getting weirdly familiar with dbconn so I can soon break it apart to work on migrations issues. This will need to move some of the db setup/migration into lib, which needs to untangle things like tracing and metric setup.

This PR is source-to-source preserving transformations to help us organize later.

Note: This is just some pre-work to get me fired up for RFC 469. Having 👀 specifically on the max_conns trickery here would be good - looks like we were setting up the pool twice and this should have the same effect as before.