[th/platform-cache-routes-2] patches related to NMPlatform cache and routes

This also introduces NMPtrArray helper. The branch uses the new type in the new function nmp_object_new_from_nl_v(). Still, the real reason why nmp_object_new_from_nl_v() is added is not yet obvious. It will be required in a future patch, where -- when we process the first next-hop of an IPv6 route -- we need to know the entire list of next hops.