Improve efficiency of grade item sortorder fixing

(Copied from MDL-43306)

Setup:

1. Create COURSE1 with default settings
2. Add an assignment 'Assign 1' with default settings
3. Create COURSE2 with default settings
4. Add an assignment 'Assign 2' with default settings
5. VERIFY: Examine the database and ensure the grade items exist, and both have the same sort order in different courses. e.g:

   SELECT id, itemname, courseid, sortorder

   FROM mdl_grade_items

   WHERE itemname LIKE 'Assign%'

   ORDER BY sortorder, courseid;

   id | itemname | courseid | sortorder

   ----+----------+----------+-----------

   4 | Assign 1 | 2 | 2

   2 | Assign 2 | 3 | 2

Attempt to create duplicates through import:

1. Go to COURSE1
2. Select import, choose COURSE2
3. Choose ONLY to import ASSIGNCOURSE2
4. VERIFY: Examine the database and ensure the grade items that are imported don't have duplicate sortorder and courseids:

   SELECT id, itemname, courseid, sortorder

   FROM mdl_grade_items

   WHERE itemname like 'Assign%'

   ORDER BY sortorder, courseid

   ;

   id | itemname | courseid | sortorder

   ----+----------+----------+-----------

   4 | Assign 1 | 2 | 2

   2 | Assign 2 | 3 | 2

   7 | Assign 2 | 2 | 4

   (3 rows)

5. Go to Grades > Categories and items > Simple view and ensure that the grade items for the two assignments appear

Backup/Restore test:

1. Backup a course with numerous activities and grade items
2. VERIFY: it restores without problem.

phpunit:
run phpunit lib/grade/tests/grade_item_test.php