Metadata files have the file handle of their source file with
0x00000001 suffixed in big endian so we can look them up directly from
their file handles.
`--nfs-cache-type symlink` is similar to `--nfs-cache-type disk` in
that it uses an on disk cache, but the cache entries are held as
symlinks. Rclone will use the handle of the underlying file as the NFS
handle which improves performance.
