Currently the only reason of refcounting rtentries is the need to report the rtable operation details immediately after the execution.
Delaying rtentry reclamation allows to stop refcounting and simplify the code.
Additionally, this change allows to reimplement `rib_lookup_info()`, which is used by some of the customers to get the matching prefix along with nexthops, in more efficient way.
The change keeps per-vnet rtzone uma zone. It adds `nh_vnet` field to nhop_priv to be able to reliably set curvnet even during vnet teardown.
Rest of the reference conuting code will be removed in the following review.