- 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

- 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 deprecated create_course_category(), change usage to coursecat. Also avoid direct DB update of table course_categories

changed to usage of
- coursecat::create()
- coursecat::update()

MDL-38147 deprecated move_category(), course_category_hide(), course_category_show(), change usage to coursecat

All usages in core replaced with:
- coursecat::change_parent()
- coursecat::hide()
- coursecat::show()

MDL-38147 Added coursecatlib_test.php, changed other tests to use coursecat::create() for creating category

Conflicts:
	lib/db/caches.php
	version.php

It was going to cause some continuous logging in the console
annoying developers on 2.5 all the time. It may be reintroduced
once filepickers are converted to proper YUI modules.

This reverts commit babf34d5.