astar_search: visitor edge_relaxed method called inconsistently before or after cost map is set

`astar_bfs_visitor` calls `m_vis.edge_relaxed` _before_ setting the new cost map value in `tree_edge` and `black_target` (and in `astar_search_no_init_tree`'s main loop from the looks of it), but _afterwards_ in `gray_target`. This is very confusing if you want to look up the new cost value _in_ your visitor method.