[FIX] point_of_sale: incorrect sequences for orders

[FIX] point_of_sale: incorrect sequences for orders

A new ir.sequence is generated at pos.config creation for the orders. However it was not used as the type was not set. The fallback was done on the general sequence.
In addition to the sequences being shared, it was not possible to create a pos.order in multicompany (no sequence found for user company, name was null).
Same issue for the pos.order.line.
This patch generates correctly pos.order and pos.order.line sequences at pos.config creation.

Instead of using the pos.config sequence to generate session number (not what this field was intended to), use the existing sequence for pos.session.
Remove company_id value on default pos.session sequence to make sure it's shared between companies and correctly set the prefix.