[FIX] event: avoid too large prefetching of event.mail

The prefetching engine wants to prefetch all the registrations to all
the events of all the schedulers, which may make a lot of data.

If the autocommit is on, then all this prefetched data is discarded at
each loop.

This commit limits the prefetching to each scheduler, so that we avoid
prefetching the data unnecessarily.

closes odoo/odoo#62042

Signed-off-by: Olivier Dony (odo) <odo@openerp.com>