Cache teachers in course

[abias]

On https://github.com/moodleuulm/moodle-block_people/blob/master/block_people.php#L106-L116, the plugin fetches the teachers everytime the block is displayed.
However, get_role_users() always triggers a DB query and does not cache the teacher lists.

It would be worthwhile to implement a cache within the plugin which caches the teacher list per course and invalidates it in case of enrolment or role changes.