Re-design stateful NAT

[qmonnet]

[mvachhar]

The main issue here is not the allocator structure or the actual rewrite, but the decision logic on deciding when to do the rewrite and lookups to find the correct allocator. Interaction with stateless NAT is also an issue. We should redesign this as we go to implement PAT so that everything is much cleaner. It can serve as the basis for the packet classification rules approach we'll need for more general firewall.