[MERGE] mail, various: improve post and notification API and performances

Purpose of this merge is to clean and improve performances of post and
notification mechanism contained in mail application.

In this merge we provide

* centralization of notification code inside mail.thread instead of being
dispatched in mail.thread, mail.message and res.partner;
* simplification of call chain, removal of unnecessary methods;
* better propagation of messages value through the various calls in order
to use given value and avoid browse and cache miss adding queries;
* optimization of attachments management in post;
* cleaning of message_post now limited to records and simplified in its
management of parameter;
* cleaning and simplification of recipients grouping for email notification;
* cleaning of message_notify;
* various code cleaning and tweaking in order to reduce query count;

Performance tests indicate number of queries did fall, especially in
use cases involving management of followers and attachments. See sub
commits for more details. This merge is linked to task ID 1943901.

closes odoo/odoo#32404

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>