* [FIX] mass_mailing_partner: Fix and optimize batch writes
Before this patch, all customizations done per contact in a batch write, except the last one, were ignored. It also was enforcing to write on some fields that were already fine, triggering their constraints unnecesarily, and causing unwanted side effects such as when combined with `mail_tracking_mass_mailing` and `mass_mailing_list_dynamic` in a fully synced list.
After this patch, customized writes are written one by one, and they only get the really needed customizations.
* !fixup [FIX] mass_mailing_partner: Fix and optimize batch writes
* !fixup [FIX] mass_mailing_partner: Fix and optimize batch writes
* !fixup [FIX] mass_mailing_partner: Fix and optimize batch writes
Jairo Llopis
David Vidal
Antonio Espinosa
Javier Iniesta