make Relation#exists? shard aware

Change-Id: Iabdb53f8a6ecfedc9c4a536aeaa5b73b7170926d
Reviewed-on: https://gerrit.instructure.com/31470
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>