Use epoch(9) for rtentries to simplify control plane operations.

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 counting code will be removed in the D24867 .

Differential Revision: https://reviews.freebsd.org/D24866