[FIX] mail: make mail_to computation of mailing traces deterministic

When having several possible emails to contact the first one is saved on the
trace model. Indeed a trace is normally linked to a unique recipient but
in case of multi-emails in an email field, several emails are available.

This commit ensure computation keeps the original mail_to when making emails
adresses uniques before sending mails and creating traces.

Followup of odoo/odoo@213d5d396c811d8e07ce928c0d31a7f47756a644

Task-2612945 (Mail: Defensive email formatting)

Part-of: odoo/odoo#133958