Allow "non core" session handler

### What does it do ?

Allows defining a path for a custom session handler.

### Why is it needed ?

We currently are able to define a custom class to act as session handler (cf. `session_handler_class` setting), but this class must be located within the `core_path`.

This PR allows defining a path (using `session_handler_class_path` setting) to load the custom session handler class.