wrap shard cache clear in after_transaction_commit
wrap shard cache clear in after_transaction_commit

fixes CNVS-22252

to ameliorate race conditions between saving and re-filling of cache

Change-Id: I7cee9a447a444e1bd84fc0fa2c16d40b35db3e69
Reviewed-on: https://gerrit.instructure.com/59637
Tested-by: Jenkins
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Nick Nelson <75ef9faee755c70589550b513ad881e5a603182c@instructure.com>

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