Exit gc NAT and static ip support

[jkilpatr]

Exits should have a config that accepts a public ipv4 subnet assigned to the router. From there the internal ip range should be split evenly across these ip, then when clients connect they should be round robined between these internal subnets to load balance them across the public ip range.

Some ip's could also be reserved to provide clients with static ip support, where that client upon connecting would have a single route for their ip.

What makes this more interesting/challenging is that exits have a large list of clients, many of which will never be seen on this exit, so we need to configure and assign ip only as clients actually show up.

This will also need dashboard support so it can be configured by operators.