[FIX] mail: Improve speed when opening systray messaging menu

Purpose
=======

Time to load systray messaging menu is too slow.

Time to load systray messaging menu:
- With 300 direct message channels, it takes at least 1.2 seconds.
- With 300 direct message channels and a dozen of "big" multi-user
  channels (#All, #News, #R&D, etc.), it takes at least 2 seconds.

The RPC 'channel_fetch_preview' is very slow. After some investigating,
it looks like 80% of the time takes place during message_format
postprocess when updating messages based on notifications:

Specification
=============

Now, we avoid to make query for each message.
And for attachment's message, we check attachment access rules and
search object only if the message has an attachment.

closes odoo/odoo#34863

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>