tnl-arp-cache: Rename module and functions to tnl-neigh-cache.

tnl-arp-cache: Rename module and functions to tnl-neigh-cache.

Since we don't distinguish between IPv4 and IPv6 lookups, consolidate ARP
and ND cache into neighbor cache. Other references to ARP related to the
ARP cache but that are not really about ARP have been renamed as well.
tnl_arp_lookup is kept for lookups using IPv4 instead of IPv4-mapped
addresses, but that is going to be removed in a later patch.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>