DA - grading rework for performance

fixes CNVS-14865

test plan:
  - with a large class
  - with DA on and off
    - check that the gradebook csv download works
      (grades can be wrong when DA is on)
    - check that gradebook2 & srgb work
    - check that student grade summary pages work

Change-Id: I7a7b4ffe302d912aa2877106295a78ab662868f1
Reviewed-on: https://gerrit.instructure.com/39470
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>