MDL-9137 various grading improvements

MDL-9137 various grading improvements
1/ initial support for migration of old grade_items and categories (not tested)
2/ rewritten grade update and calculation logic
3/ initial support for calculation formulas
4/ minor API refactoring and cleanup
5/ various small bugfixes
6/ fixed use of grademax with scales
7/ fixed some unit tests

TODO:
* implement proper locking of grades - needs discussion
* force recalculation of all formulas after adding/removing/changing of grade items
* better delete flag support
* support for NULLs n backup - Eloy already proposed a solution
* support for NULLs in set_field()
* speedup
* more unit tests nd functional tests