Improve or replace PANE Discovery Protocol

[adferguson]

PANE currently features it's own network discovery protocol. While it currently appears to get the job done, we should look at whether it has bugs and should be fixed, or consider replacing it with discovery abilities already built into OpenFlow.

NOX includes this functionality as well -- we should look at their code for guidance. From a message on OpenFlow-discuss:

You need to run the discovery application. That application will send / receive LLDP messages on all switch ports to detect the topology. It will generate "Link_event" events [1], notifying you of the detection of a link between two DPID/port pairs.

You should also listen to the regular Datapath_join_event and Datapath_leave_event to figure out which DPs are there.

[1] https://github.com/TrafficLab/nox11oflib/blob/master/src/nox/netapps/discovery/link-event.hh

[adferguson]

and this is Pox's implementation of LLDP: https://github.com/noxrepo/pox/blob/master/pox/openflow/discovery.py