Add VXLAN backend

VXLAN datapath is supported by the kernel thereby reducing the
overhead of TUN device and userspace switching. This patch takes
advantange of DOVE extensions and does not use multicast for port
flooding.

Fixes #18
can't you do a double `go`-routine here?

eg, go to a first level goroutine select over `misses` and a `shutdown` channel.  On shutdown, it can actually call `Close()` on the Netlink file descriptor?