Smarter NeighborCache state machine.

[dlrobertson]

RFC 4861 7.3.2 defines several states a Neighbor Cache entry can be in. Neighbor Cache entries should be updated to include these states and the use of the Neighbor Cache entries in the EthernetInterface should be updated to handle these new states.

[dlrobertson]

@squidarth since you've already done some work on the neighbor cache, this could be a good follow-up issue. It will be a good bit more work as the use of cache entries in EthernetInterface will need to be updated, but I think it is a reasonable next issue to work on, once #234 is merged.