Incorporate latency calculations

[dnwiebe]

This card depends on GH-586.

We don't have time for a complete description of the problem here, but this card should be about modifying the existing latency-measurement logic to:

1. Be triggered whenever an appropriate event occurs (perhaps every so many seconds or minutes; perhaps whenever a new Node joins the network; perhaps when Gossip carrying a change to the network arrives; perhaps something else).
2. Modify the Neighborhood when a Ping response arrives by updating the latency estimates on each affected link
3. Modify the routing engine to take computed link latencies into account when computing route undesirabilities.