This is a pre-requsite to enable nhop-based route operations required
for efficient multipath control plane support & netlink multipath
support.

• add explicit basic low-level radix operations: add_route() (renamed from add_route_nhop()) delete_route() (factored from change_route_nhop()) change_route() (renamed from change_route_nhop)
• remove "info" parameter from change_route_conditional() as a part of reducing rt_addrinfo usage in the internal KPIs
• add lookup_prefix_rt() wrapper for doing re-lookups after RIB lock/unlock

MFC after: 2 weeks