Extract top-level functions for session initialisation

The session initialisation has too many implicit dependencies. To break
these apart, we extract local functions and turn them into top-level
definition with all parameters explicitly given.

This commit only makes sure session initialisation functions are
promoted to top-level definitions and tries to simplify them.
The top-level definitions are lacking type signatures to make it easier
to change them, but we plan to add them back.

Author: fendor