libceph: introduce decode{,_new}_pools() and switch to them

Consolidate pools (full map, map<u64, pg_pool_t>) and new_pools (inc
map, same) decoding logic into a common helper and switch to it.

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