1. I've moved session init to separate class (out of uploader.php) because I think that starting and managing of session is not in uploader's responsibility.

Important: return $_CONFIG; at the end of config.php file is necessary. I think that use of $config = require 'conf/config.php'; is better and more clear than require 'conf/config.php'; $something = $_CONFIG['key'].

2. It still supports "session key" definition in the config file but now also \Iterator objects or clean arrays.

So we can set _sessionVar in the config like KCFINDER and SESSION['KCFINDER'] will be used.

But we can also set $someObject as value of _sessionVar. Then this object will be used for sessions.
It's very useful for better session control for example with frameworks. In this case, KCFinder won't start session and won't access $_SESSION super global variable directly. It will access via given object from config.