Summary:
Previously, it's fine to migrate up to metalog, and fine to migrate down once
since we double write data. However, re-enabling metalog can be problematic
since there is no code path to do "migrate up" again.
This diff fixes the issue by tracking what files (or "keys") are using metalog
as the source of truth in metalog. So we can accurately figure out whether to
migrate svfs files to metalog on demand.
Reviewed By: xavierd
Differential Revision: D18864424
fbshipit-source-id: e61e1790c231f9c88de869f413f27bb954a29920
