batman-adv: make struct batadv_neigh_node algorithm agnostic

some of the fields in struct batadv_neigh_node are strictly
related to the B.A.T.M.A.N. IV algorithm. In order to
make the struct usable by any routing algorithm it has to be
split and made more generic

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>