Janitor fails to delete blobs because directory is not empty

[Jared Neil]

WRN Error deleting blob (03516230-c019-4f85-8283-1059f6512a4a): remove /buildbuddy/03516230-c019-4f85-8283-1059f6512a4a: directory not empty

I think this has something to do with how Chunkstore and DiskBlobStore interact, but I think it comes down to
DiskBlobStore.DeleteBlob calls disk.DeleteFile, which calls os.Remove, which always fails because the blob is actually a directory, and not empty.

[Tyler Williams]

Hi @JaredNeil, sorry for the delay on this, but the fix is now submitted and will go out in next week's release :)