VXLAN static forwarding

This removes the complicated monitoring code and replaces it with three network configuration entries for each subnet lease that cause the packets to end up in the right place:

- A route pointing all traffic to the subnet via a gateway that is the IP of the flannel interface on the target host.
- A forwarding database entry that maps the network routable IP address of the target host to the MAC address of the flannel interface on the same host.
- An ARP table entry mapping the IP address of the flannel interface on the target host to the MAC address of the same interface.