make the root account cache relative to the default shard, not the birth
make the root account cache relative to the default shard, not the birth

refs CNVS-36971

this means if the account is already cached, and Rails hasn't loaded column
info yet, the metadata query will happen against the default shard, not
the birth shard

Change-Id: Icfd98fb685a890b35446ebc841ee1a315b49ae9b
Reviewed-on: https://gerrit.instructure.com/113083
Tested-by: Jenkins
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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