gt: expire Ethernet cache of neighbors

Instances of the GT block maintain a cache of Ethernet header caches; see `gt/main.c:gt_neigh_get_ether_cache()`. While this cache removes entries when it's full (see `drop_cache_entry_randomly()`), it should also have an expiration of those entries as well. Whenever those entries are looked up (see `lookup_ether_cache()`) the expiration field should be updated.

This is important because IPv6 prefixes are so huge that they can easily be much larger than the entire cache. While the size of the cache can easily match the number of active IPv6 addresses, it can't match the size of an IPv6 prefix /64.