Add vxlan tunnel monitoring example

Add vxlan tunnel monitoring example

* Example simulates many vxlan hosts ("hypervisors") with multiple clients per
  host. Some clients will be sending traffic to another client on a different
  host.
* Add vxlan header to proto.h
* Remove unused import line in vlan_learning
* NOTE: to display this demo, it is required to checkout the chord graph demo
  from https://github.com/drzaeus77/chord-transitions.git.
  Follow the instructions in that repo to get a simple http server up and
  running.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>