minimise the impact of invalidating routes on topology updates

[murali-reddy]

When there is change in the topology due to addition/deletion of the nodes or transient failures of established connections to the peers, all ODP flows are deleted by InvalidateRoutes

https://github.com/weaveworks/weave/blob/v2.5.2/router/network_router.go#L70
https://github.com/weaveworks/mesh/blob/v0.2/routes.go#L216

As the topology updates are broadcast to the peers it result in routes calculation multiple times, hence performing InvalidateRoutes

As data point one of the weave-net pod logs of 40 nodes cluster shared in #3641. When a node got disconnected it resulted in performing execess of 100 InvalidateRoutes

weave-net-vjrvd

This invalidating routes would occur across the cluster.

How does this impact? While flows are eventually recreated on the nodes, it leaves window of time where traffic is largely broadcasted.

This feature request is to implement a InvalidateRoutes() in which flows are deleted only if they are impacted by topology change.