MDL-9506

MDL-9506
grade_grades_raw
    added grade_item object
    added load_grade_item()
    added call to grade_item->flag_for_update() when object is inserted, deleted or updated
grade_grades_final
    added grade_item object
    added load_grade_item()
grade_grades_text
    added grade_item object
    added load_grade_item()
grade_item
    added qualifies_for_update()
    added call to grade_category->flag_for_update() when object is inserted, deleted or updated (only when qualifies_for_update() is true)

However since I have made this last set of changes, the unit tests are performing extremely slow, and several of them fail. I suspect that there is a serious performance issue involved in using objects too heavily, instead of performing more SQL queries, especially when updates are concerned. Often a complete object is SELECTed before it is amended in an object form then UPDATEd.

In the next iteration I will look at this seriously, and put together some tests with thousands of users and many grade_items, to see how well the API responds.