optimise Peer.Routes() for fully connected mesh of node topology

[murali-reddy]

Routes calculations is order of O(n_peers^2) was optimised in

weaveworks/weave#1773
weaveworks/weave#1761

However for use-cased where mesh of nodes are fully connected i.e) each node is single hop away. Consider this topology as special case when calculating the routes.

This topology is very common in case Kubernetes clusters, and weave-net consuming the mesh library can benefit from this optimised routing.