Computation of the last post shouldn't be done with a
db request for each forum but can be batched.
Technical note:
As it is likely that `post_ids` is not yet fetched when we're
accessing `last_post_id` we chose not to early return if no
forum in `self` included posts already.
Version choice:
We chose this version to merge this as it is the one where
forum was taken over, and _read_group refactored.
A test for this computation is also added.
Task-3685002
closesodoo/odoo#150444
X-original-commit: ba3af942deee4818edb32d08b9bb07652253a029
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>
Signed-off-by: Florian Charlier (flch) <flch@odoo.com>