Incorrect implementation of load_many_for_cache() in multiple places

There are a number of places in core that implement load_many_for_cache() incorrectly:

\core_message\time_last_message_between_users
\mod_assign\cache\overrides
\mod_lesson\cache\overrides
\mod_quiz\cache\overrides

Specifically, they all use the same code:

        foreach ($keys as $key) {

            $results[] = $this->load_for_cache($key);

        }

The problem is that $results needs to be indexed by key. I would also note the the documentation around it so not clear, which likely leads to the issue.