Vpn performance

This request contains the changes I've mentioned in the thread
https://groups.google.com/d/msg/snabb-devel/eOMS8zZWi90/wEVbd3C-8DUJ

The major performance boosts come from the recycling of objects in the protocol header mechanics through my class.lua thingie and the use of libpcap for pattern-matching of incoming packets.

This code performs at line-rate up to almost 700 kpps in my test setup. The limiting factor there are the hosts that run iperf, not the VPN boxes.