MDL-51305 grade: Fix slow grade operations by setting grade_item early.

Exports were fetching the grade item for every item-user combination.
Setting the grade_item object on the grade_item early prevents that.

grade/lib.php

@@ -308,6 +308,7 @@ public function next_user() {
                     $grades[$grade_item->id] =
                         new grade_grade(array('userid'=>$user->id, 'itemid'=>$grade_item->id), false);
                 }
+                $grades[$grade_item->id]->grade_item = $grade_item;
             }
         }