Vpn performance

This request includes the performance hacks that I've described in
https://groups.google.com/d/msg/snabb-devel/eOMS8zZWi90/wEVbd3C-8DUJ

The most relevant commits are https://github.com/alexandergall/snabbswitch/commit/2b6e39ef8eec345737035886ae87c5f5091a6d97 and https://github.com/alexandergall/snabbswitch/commit/89e88deed0ced0825e76ae3a3fa25c4381ab4f98

The object recycling technique makes the OO approach feasible, at least up to the performance in my test setup, i.e. close to 700 kpps (the limiting factor in this test were the end systems running iperf, not the VPN app). Whatever that's worth :)

The "matcher" module is at the proof-of-concept level, but appears to be a possible way forward for efficient packet classification.