move instructors enrollments query to slave
move instructors enrollments query to slave

closes TALLY-803
flag=none

Test Plan
- Create an assignment.
- As a student, submit to the assignment.
- Re-submit to the assignment.
- Leave a comment on the submission.
- Verify that the instructor of the course received messages for all
three events.

Change-Id: I33ff1b34682c886fc8fac709dba221dcefc7233e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/232856
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Gary Mei <4dff5ebcff9ed76793b70b9470ac64791a16ed11@instructure.com>
Product-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms