Summary:
The patch adjusts the definition of BlobDB's DB properties a bit by
switching to `GetBlobFileSize` from `GetTotalBlobBytes`. The
difference is that the value returned by `GetBlobFileSize` includes
the blob file header and footer as well, and thus matches the on-disk
size of blob files. In addition, the patch removes the `Version` number
from the `blob_stats` property, and updates/extends the unit tests a little.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8902

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D30859542

Pulled By: ltamasi

fbshipit-source-id: e3426d2d567bd1bd8c8636abdafaafa0743c854c