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>
ceph_test_rados: don't update any state on successful cache-evict

- we didn't touch the user_version
- we didn't change the clean/dirty state

Signed-off-by: Sage Weil <sage@inktank.com>
ceph_test_rados: test flush/evict on snaps

Signed-off-by: Sage Weil <sage@inktank.com>