[gold] Implement a dual clstore
[gold] Implement a dual clstore

We should be able to put the firestore impl as primary
and the sql impl as secondary as we migrate data and then
swap primary and secondary. After that looks good, we
can remove this and just go with the SQL as the only clstore.

Bug: skia:10582
Change-Id: Ib42b54ee11a898af79bc804d6bfba08c071567b2
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/353704
Reviewed-by: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>

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