[FIX] uom: _check_category_reference_uniqueness

Edit the liters uom (default in its category) and change its category to
unit. You now have two references uom in the unit category. Try to set a
third one and now it fails.

This was because the constraint is implemented with an sql query but
since the new ORM the update in database are delayed. We force a
database update with flush so that the query gant get all the
informations it needs.

closes odoo/odoo#41621

X-original-commit: 68ced797
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>