IPv6 relay support

[hetelek]

Requires these changes: pion/turn#265
I've only tested on Linux with UDP turn servers.

This adds support for IPv6 relays. When discovering candidates, Pion will attempt to contact the turn servers via both IPv4 and IPv6. You can also specify a IPv6 literal address in the config, such as:

{
    "iceServers": [
        {
            "urls": [
                "turn:[2001:0db8:85a3:0000:0000:8a2e:0370:7334]:3478"
            ],
            "username": "user1",
            "credential": "password1"
        }
    ]
}

[hetelek]

Previously, network was hardcoded to NetworkTypeUDP4 even if the url.Proto == ProtoTypeTCP.
Not sure if this was correct?

[hetelek]

What you do here is that you have the user decide whether to use IPv4 or IPv6. Then, you try judst one address of the relay, either the first IPv4 address or the first IPv6 address.

This requires no extra configuration from the user. The example in the description shows that a IPv6 address can be used, but it's not required. If a hostname is provided (e.g., turn:turnserver:3578), then Pion will attempt to gather candidates from it via both IPv4 and IPv6

[jech]

I stand corrected. The iteration happens at a higher level than I was expecting.

I need time to grok the fullness.

[jech]

I strongly support this work. IPv6 support is necessary for Pion, it greatly simplifies connectivity in many cases (e.g. in Docker containers).

This version works by gathering two candidates for each double-stack TURN server, one for each address family. Two comments:

1. For TCP servers, this might be overkill ­— it should be enough to call net.Dial("tcp",...), and have the network stack perform ordinary probing for listening addresses.
2. For UDP servers, this might not be enough — the server may have multiple IPv6 addresses, only some of which are reachable at a given time. Shouldn't we be generating one candidate for each address?

[hetelek]

2. Shouldn't we be generating one candidate for each address?

Yes, that sounds like a better approach. I'm thinking we should limit the max amount of addresses just in case the DNS lookup returns many IPs. For example, max 5 IPv6 and 5 IPv4 addresses per relay.

1. For TCP servers, this might be overkill

I really don't know how TCP works here, so looking for guidance. It's not clear to me why we would force IPv6/4 for UDP but not TCP.

Also, will this break TLS since we will be passing IP addresses instead of hostnames? With the web/https, the hostname is checked against the cert's common name. Not sure if that's also applicable here.

[stv0g]

Can we separate the retry logic into a separate PR? I think its unrelated to the IPv6 support..

Also pion/turn#265 might get obsolete if pion/turn#276 gets merged.