Implement LRU-based replacement for neighbor-attr

Added a last_lookup field to neighbor_attr allowing to remove oldest entry before inserting a new entry in a full table. Added an explicit neighbor addition in neighbor-info even in case of failed transmission, so that bad links are remembered.

These are fixes that were needed to get ContikiRPL to perform a reliable data collection in 130+ nodes testbed. The node density was exceeding 30 at some nodes, revealing the need for a replacement policy in neighbor_attr. Tested successfully in the testbed with neighbor tables of 16 entries (neighbor_attr, ds6, and rpl parents).