libceph: primary_affinity decode bits

Add two helpers to decode primary_affinity (full map, vector<u32>) and
new_primary_affinity (inc map, map<u32, u32>) and switch to them.

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