instructor_task: Distribute extra items among subtasks of last query.

When creating an instructor task total_num_items may change between the
time it and the number of subtasks is calculated and the time the
subtasks are actually queued (all of this cannot happen in one transaction).
In such a case the extra items are distributed among the subtasks of the
last query.

LMS-2090
Merge pull request #2396 from edx/usman/lms2090-instructor-task-transactions

instructor_task: Distribute extra items among subtasks of last query.