Checkpoint progress on "worker_checksums_verify_file"

This is a "File-orientated" variant of worker_checksums_verify.

The idea here is that a path to a file in the data directory can be specified
on disk, and the verification will make the appropriate lookups to verify the
minimal relevant relation fork (and not associated objects/files, as with the
original). This is likely to be more useful for low level tools than having to
specify a regclass directly.

This doesn't work yet, but with this commit, much of the infrastructure is in
place.