Implement l2fwd-like app

[wingo]

We got reports that a user was able to process traffic with the DPDK but not with Snabb. The Snabb counters were all zero. This kind of problem is hard to debug remotely and probably an issue with the upstream intel 82599 drivers. We should implement a simple l2fwd-like app and submit it to upstream Snabb, to use as a first step when troubleshooting connectivity problems. We can also use this app for benchmarking.

See http://dpdk.org/doc/guides/sample_app_ug/l2_forward_real_virtual.html for a description of l2fwd.

[wingo]

See snabbco#681 (comment) for some more comments.

[wingo]

Ideally this app could run on virtio or tap interfaces as well.

[dpino]

Proposed PR snabbco#736