librbd: drop unused elapsed calc for diff_iterate

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

Signed-off-by: Sage Weil <sage@inktank.com>
librados: cleanly define SNAP_HEAD, SNAP_DIR constants

We were using the internal CEPH_NOSNAP and CEPH_SNAPDIR constants, and
defining a clone_info_t::HEAD (with a different value).  The docs were
referrring to the internal constant names.

Instead, define librados constants (C and C++) with the same values as the
internal types.

Note that this changes the clone_info_t::HEAD value from -1 to -2 so that
it now matches the internal type.

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