[BEP32] Implement IPv6 DHT support

[alanmcgovern]

This implements BEP32 by hosting two independent DHT stacks. This can be augmented in future to allow returning IPv4 responses to IPv6 queries, and vice versa.

Features:

• a node should use the same node id in both tables
• Do not send UDP datagrams with a payload larger than 1024 octets
• IPv6 DHT should use a socket bound to one of the host's global unicast IPv6 addresses
• The port message from BEP5 is set to the IPV4 dht port, or IPV6 dht port, depending on which protocol is used to send the message.
• values should be a list of compact ipv4 peers, or a list of compact ipv6 peers.
• FindNodesResponse and GetPeersResponse should (optionally?) support nodes6
• FindNodes and GetPeers should support the want parameter, a bencoded list of n4 and/or n6. This controls whether to include a nodes or nodes6 response.
• If there's no want parameter, return nodes matching the request addressfamily.
• values must only contain peers whose contact information matches the request addressfamily