sheep: cache sha1 digest for read-only objects

The contents of read-only objects are immutable.  We can cache their
sha1 digests as extended attributes for the next sha1 calculation.
With this patch, I can complete 'collie vdi check' of a 4 GB snapshot
volume in 0.2 seconds.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>