Add specialized records methods, and find_to_populate_by_keys

[lgebhardt]

Specialized records methods allow the system to avoid duplicating
expensive permission checks for records that have already been found.
This is needed because of the new multiphase approach building up the
result set.

WIP for testing and wording review. I think this needs tests to ensure correct usage, but I'm not sure of the best way to handle this.

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