[IMP] point_of_sale: reconcile payment

When a PoS order is closed, the payment and the sales journal item are
not automatically reconciled. This potentially leaves a lot of
unreconciled entries for a customer, which will appear in his follow-up
report.

This introduces an auto-reconciliation process upon order closing. The
process restricts the reconciliation to an order individually, i.e. we
don't try to cross-reconcile several orders for a customer.

opw-697565