Use routeresync to Swap the Routing Stack

We have demoed the data plane uninterrupted routing stack swap with our simple utility routeresync. Here are the steps:

1. Run the command routeresync start. This step freezes the current data plane forwarding table so that during our control plane modification, the data plane won't be touched.
2. Remove the previous routing stack and run the command 'ip route flush' to flush all the previous routes.
3. Install the new routing stack and wait for all the new routes to be learned.
4. Run the command routeresync stop. This step unfreezes the current data plane forwarding table and then the orchagent will sync the differences between the previous routes and the current routes.