Separate weekly tasks for tarball-integrity checks.

[isoos]

• Split integrity check into multiple process and run them independently. #8848
• Refactored the IntegrityChecker to have a base functionality and split out tarballStorage-related checks. This reduces the time it takes to run the regular integrity checks, and it is also a natural split point for the future SQL migration.
• Also added further cross-reference in the other direction: that each file in the canonical storage should have a matching database entry.
• Note: I think the canonical bucket's cross-reference is a good test, but I'm not sure if and how many stale uploads we had that left an archive there. At least this will get us some insight about that through the logs.
• We could also consider a similar scan for the public bucket, though it seems to be not critical, since the update of that bucket is always predicated on the canonical one.