[IMP] point_of_sale: avoid writing on pos config when session open

When a session is opened on a POS config, we don't want to let users
editing the pos config as modifying journals, payment method,
pricelists,... Could lead to inconsistancies between what has been
encoded on the pos orders and what are the available values on pos
config.

Before this commit, it wasn't taking into account the rescue sessions,
only the normal ones.