touch the course in grade calculator less often

We invalidate the course cache by touching the course too often. This
can cause issues in the database at scale. This patchset only touches
the course once after all the calculator work is done.

closes TALLY-849

test plan:
 - specs pass

Change-Id: Id006b9618ab89c003bd2d4b354247036dca9ce09
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/234082
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jeremy Neander <jneander@instructure.com>
Reviewed-by: Spencer Olson <solson@instructure.com>
QA-Review: Keith Garner <kgarner@instructure.com>
Product-Review: Keith Garner <kgarner@instructure.com>

lib/grade_calculator.rb

@@ -91,6 +91,9 @@ class GradeCalculator
     user_ids.sort.in_groups_of(100, false) do |user_ids_group|
       GradeCalculator.new(user_ids_group, course, opts).compute_and_save_scores
     end
+
+    # Touch the course to naively expire the cache
+    Course.where(id: course_id).not_recently_touched.update_all(updated_at: Time.now.utc)
   end
 
   def submissions
@@ -515,8 +518,6 @@ class GradeCalculator
     return if joined_enrollment_ids.blank?
     return if @grading_period && @grading_period.deleted?
 
-    @course.touch
-
     save_scores_in_transaction
   end