libceph: support CEPH_FEATURE_OSD_CACHEPOOL feature
libceph: support CEPH_FEATURE_OSD_CACHEPOOL feature

Announce our (limited, see previous commit) support for CACHEPOOL
feature.

Signed-off-by: Ilya Dryomov <33f1a10eef0a3354755a1541edfdb100ff87fcfc@inktank.com>
Reviewed-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@inktank.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs