This PR fixes several issues with the unread counter:
- Notifications being considered as unread messages (which is not
the case on the server side which resulted in inconsistencies).
- Counter greater than it should be when the last seen message is
not loaded.
- Messages not marked as read on first thread opening.
task-3239957
closesodoo/odoo#116208
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>