MDL-38147 event-driven cache purging for course categories
MDL-38147 replaced function coursecat::get_all_ids() with coursecat::get_tree()
MDL-38147 coursecat: bug fix in caching contexts
get_children
test for get_children
MDL-38165 cache: fixed up session event purging
MDL-38165 cache: fixed up application event purging
more tests