ceph_test_rados: test read from snapshots

This was disabled back in 2011, c54aa7db3bc6e4c763e3b08d2ae98f89afe5a246.
Whoops!

Signed-off-by: Sage Weil <sage@inktank.com>
osd: preserve user_version in snaps/clones

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1004 from ceph/wip-snaps

make ceph_test_rados read from snaps; resulting bugs found

Reviewed-by: Loic Dachary <loic@dachary.org>