cbbackupmgr info should use the file size stored in the stats files

As described in MB-37655 we are adding additional information to the ChunkStats; we are including the size of the files on disk. Info should read from these files instead of manually calling 'os.Stat()' on each file, however, for backwards compatibility it should be able to fall back to using 'os.Stat()' in cases where the file size was not included in stats files.