RDB Loader: migrate to slf4j logging

[chuwy]

Currently we use our own Logging algebra, which allows us to test logging and abstract it away, however it's very unflexible.

1. It doesn't allow to disable logging for some classes at launch time
2. It doesn't show where logging is coming from
3. It doesn't allow to configure severity

log4cats gives us these abilities, but unfortunately has another downside - we need to either abandon F[_]: Logging signatures and instantiate one logger per class (and damage testing) or have Main class everywhere, which defeats 1st and 2nd points.

There's some WIP in log4cats I hope to get merged at some point (typelevel/log4cats#421, typelevel/log4cats#397). When the PR is merged we can migrate without trade-offs.