libceph: add ceph_pg_pool_by_id()

"Lookup pool info by ID" function is hidden in osdmap.c.  Expose it to
the rest of libceph.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>