MDL-38147 Performance improvements to coursecat class:

- Retrieve and cache only often-used fields of course category
- Removed function coursecat::get_all_visible() as potentially causing performance issues
- removed function coursecat::get_all_parents() as ineffective and unnecessary, replaced with get_parents()
- retrieve all fields from course_categories when unretrieved field is accessed

Also some code improvements:
- rename functions starting with _ , rename arguments, etc.
MDL-38147 Improvements to caching of course categories, coursecat::get_children() improvements

- Course categories caches are purged by event now
- session cache has additional 10 minutes ttl to clear itself and accomodate for permission changes that do not trigger event purging
- additional request-level cache for coursecat::get()
- We store only children of one category in one cache key
- Function coursecat::get_children() can return results sorted and/or paginated. Added tests
MDL-38147 changed logic for building list in make_categories_list()
MDL-38147 fixes to phpdocs and avoid using deprecated context functions
MDL-38147 Added functions coursecat::search_courses(), get_courses()

Also search_courses_count(), get_courses_count();
Added unittests