Limit the amount of active fallback peers

[LeeSmet]

Currently we keep all fallback peers which are feasible, and also infeasibles as well (if they have been retracted recently). Certain network topologies can however cause this to become a rather large list. It might be useful to limit the amount of (fallback) routes for a subnet to some number, e.g. 5 should be more than enough (10 if we are really conservative), and kick out the furthest (distance wise) peers. This does mean we need to take care to handle new peers, since those could potentially become the best in the future but start out as the worst and get kicked out. Some grace period should be taken into account for how long we know the given route, to consider if we want to remove it.