VPN performancev2

This set of changes implements the performance improvements of the lib/protocol framework and the VPWS app that I've outlined on the mailing list some time ago.   The most important improvements are

   -- a "recycling" mechanism for object instances through the "class.lua" interface
   -- a binding to libpcap for efficient packet matching

The run time behaviour of apps/vpn/vpws.lua still exhibits various different states regarding the percentage of compiled, interpreted and GC code when run repeatedly, which I find hard to understand.  The code changes in other parts of the Snabb Switch influence this as well.