[FIX] point_of_sale: duplicate orders

When you create a pos order, it is possible that the request timeout or
the client loose the connection with the server, that will lead to not
have the server_id of the order sent to the server. So when the client
will try to synchronize the order again, it'll not send the server_id to
the server, and the server will create a new order which is a duplicate
of the one created when the client lost the connection.

To avoid this duplication, we are using the pos_reference to find if
there is an existing order like in 12.0 to avoid duplication.

closes odoo/odoo#40837

Task-id: 2127656
X-original-commit: cddf29e7
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>