In development, when code is reloaded, we lose the connection class
attribute. So we need to setup a to_prepare block to reconfigure DB load
balancing.

Also add logging for when DB load balancing setup runs.

Note: whether code reloads depends on config.cache_classes,
and config.reload_classes_only_on_change. In production,
config.cache_classes is true, so no code reloading happens there.

Changelog: fixed