Sybil selection should prefer measured over advertised bw

When choosing between two nodes on the same IP, we based the choice on bandwidth. But right now, we use dirserv_get_bandwidth_for_router(), which looks at measured bw and falls back to advertised when measured isn't present. Probably it makes more sense, if there are two nodes, one of which has measured bw and one of which doesn't, to prefer the one with measured bw.