[FIX] website : prevent bad query

In database with front end, there are lot of bad query like this:
bad query: UPDATE "website_visitor" SET "timezone"='Europe/Paris',"write_uid"=116,"write_date"=(now() at time zone 'UTC') WHERE id IN (15414)

closes odoo/odoo#70507

Error: Could not serialize access due to concurrent update
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>