[session] gather close_cnx with get_cnx and set_cnx They do the same kind of operation and deserve to be grouped together.

[client-connection] remove the _srv_cnx usage It does not do anything special now that we use explicite Connection object with automatic cnx_set handling.

[client-connection] explicitly check that the client-connection is open The check is also perform by the _srv_cnx property. But we do not need those property anymore.

[client-connection] handle the lack of connection id while not open the connection id is computed at opening and forgotten when closing. We can't rely on it in various messages … like the "connection closed" exception.