[IMP] core: write x2many fields last

Writing on x2many field should be done last, because deleting x2many
lines causes pending computations and updates to be flushed.  Writing on
column fields after that inevitably adds extra update queries.

We introduce an attribute `write_sequence` on fields to order fields for
write.  The prescribed order is: all fields except monetary and x2many,
monetary fields, x2many fields.

closes odoo/odoo#65959

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>