Move `<add|del|change>_route()` functions to route_ctl.c in preparation of multipath control plane changed described in D24141.
Currently `route.c` contains core routing init/teardown functions, route table manipulation functions and various helper functions, resulting in >2KLOC file in total. This change moves most of the route table manipulation parts to a dedicated file, simplifying planned multipath changes and making `route.c` more manageable.