Draft: POC: Use ConnectionHandler to implement transparent connection proxy

What does this MR do?

I was wondering how differently we could enable support for LB in a transparent way to work across any type of connects_to : specification.

This tries to implement a ConnectionHandler which does create a connection proxy for the purpose of LB depending on a configuration.

This appears to work for me transparently to support any number of connection proxies.

The SD should work for main database with these changes.

Ah.: We need to pass actual db_config as well down to LB/ConnectionPool that we know at this moment.

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