shardify db tasks in rails 3

refs CNVS-13741

Change-Id: I36a3f6d2995681d72940fbc15d2548f548611a60
Reviewed-on: https://gerrit.instructure.com/36896
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>