Upcoming Session Router usage will use IPv6 (internally) to talk to
relays, but currently relays are only listening on the IPv4 0.0.0.0 any
address.  This adds a second HTTPS and QUIC listener on the IPv6 any
address (::) so that it will be accessible over session router.

- Updates HTTP server internals to properly handle IPv6 addresses,
  including updating the rate limiter to handle them.

- Switch to using a dual-stack socket with uWebSocket (HTTP) because it
  forces dual stack on, and was failing to bind with separate IPv4/IPv6
  sockets.

- Make QUIC match dual-stack (although it doesn't strictly need to) for
  the sake of consistency.

- Change internals for request handling to use IPv6 for requests and
  rate limiting, with IPv4 addresses using mapped IPv6 addresses.

Make HTTP & QUIC listen on IPv6