Fixes#40060
* Fix coding style errors
* Use CONNECTION LIMIT (no underscore)
* From review done by amenonsen and bcoca - Set default at None, make the change detection less confusing
* Added EXAMPLE on how to apply a database specific connection limit
* Added some basic tests for conn_limit applied to a database
* Check that conn_limit has actually been set / updated to 200
* Add changelog fragment regarding postgresql_db conn_limit parameter
