Currently, if we create a new event mail "after each registration" to be send
directly, the next time we confirm a registration it tries to send the
communication to all registrations that are not yet contacted. Indeed
the first new registration triggers the scheduler which runs on all
pending registrations. This may potentially break the transaction or
at least make it extra slow.
Now, we send the mail only to the new registrations (created after the
event mail). Old registrations will be contacted via the CRON instead.
Task-3084943: Event: Improve communication scheduler scalability
Part-of: odoo/odoo#155777
Co-authored-by: Stéphane Debauche <std@odoo.com>
