Cache role names in course

[abias]

On https://github.com/moodleuulm/moodle-block_people/blob/master/block_people.php#L121, the plugin fetches the role names everytime the block is displayed.
However, role_get_names() always triggers DB queries and does not cache the role list.

It would be worthwhile to implement a cache within the plugin which caches the role list per course and invalidate it in the case if a) the admin changes the plugin configuration and wants to display other roles or b) if the roles get renamed globally or c) if the role names are overwritten in the course settings.