[IMP] mail: reduce message_post query count and clean code

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