Purpose of this commit is to improve performance of post and notify through
various code improvements.
Containing
* making of _notify_customize_recipients a mail.thread method;
* _message_auto_subscribe_followers optimized;
* renaming and optimisation of _notify_partners;
* message_post_attachements optization;
* remove unused mail_post_autofollow_partner_ids context key;
* minimize message create queries;
* only check record related access right for thread messages not in pending
moderation;
* get attachements from values instead of db;
* storing lang in ctx from the beginning;
* don't track on write on attachments;
Related to task 1943901
Linked to PR #32404
Xavier-Do
de7c119e66