libceph: add support for osd primary affinity

Respond to non-default primary_affinity values accordingly.  (Primary
affinity allows the admin to shift 'primary responsibility' away from
specific osds, effectively shifting around the read side of the
workload and whatever overhead is incurred by peering and writes by
virtue of being the primary).

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>