discovery: Always eat LLDP; add eat-early-packets

The discovery PacketIn handler now always eats PacketIn events
that it thinks are its LLDP packets.  Additionally, there's an
--eat-early-packets option now.  When active, this will block
pretty much all PacketIn events until a switch has been
connected for a complete discovery cycle.  To accommodate
these changes, discovery now listens to OpenFlow events with
very high priority.
core: More flexible args in listen_to_dependencies

You can now specify a None key in the listen_args map.  Entries
in this map will be used for ALL components unless the individual
component overrides them.