use loose singleton for recache_grade_distribution
use loose singleton for recache_grade_distribution

Change-Id: Ie8d7c0080e2dd10a30ffcddffa6f1c8cf355c9bc
Reviewed-on: https://gerrit.instructure.com/c/analytics/+/236846
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

instructure/analytics,instructure/analytics,instructure/analytics