MailActivity model has an ``_action_done`` method that is quite heavily
used and overridden. It notably posts messages, create next activities but
also has some specific behavior depending on category.
Purpose of this commit is to improve code of this method to avoid useless
queries, improve performance, and rethink the computation with batch in
mind.
Task-2883589 (Activity performance and cleaning)
Part-of: odoo/odoo#93682