optimize cross-shard conversations query
optimize cross-shard conversations query

don't bother querying if we know there won't be relevant data on that shard

Change-Id: I76ed81b0b6257dfa18c229bfff6d9812ee3e8710
Reviewed-on: https://gerrit.instructure.com/71260
Reviewed-by: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Tested-by: Jenkins
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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