ceph: shared persistent read-only rbd cache

This patch introduces introduces RBD shared persistent RO cache which
can provide client-side sharing cache for rbd clone/snapshot case.

Change-Id: Icad8063f4f10b1ab4ce31920e90d5affa7d0abdc
Signed-off-by: Qiaowei Ren <qiaowei.ren@intel.com>
Signed-off-by: Dehao Shang <dehao.shang@intel.com>
Signed-off-by: Tushar Gohad <tushar.gohad@intel.com>
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Signed-off-by: Yuan Zhou <yuan.zhou@intel.com>