support a staging bucket in bulk storage

[isegall-da]

Part of #6162
(See #5884 for context)

Could benefit from some more unit testing of the update history part, but this PR is a behemoth as it is (sorry, couldn't break it in a very sensible way), so I'm leaving that as a followup PR (and hence not competing #6126 here)

Note that the changes are not bwd compatible, so require disabling bulk storage and deleting the data on CILR&DevNet where we enabled it (tracked in #5844)

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Cluster Testing

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