POC to close #469 - Establish connection to the correct pool when using a replica

[weilandia]

closes #469

Currently, when establish_connection is called in sql_adapter.rb it connects to the given URL via the primary connection pool. By passing the config name to establish_connection instead of the url, we can avoid this.

I can do more work on this if this is the direction you'd like to go, but wanted to put up a POC first.