Perform on-join DNS requests over TCP about memberlist

[armon]

Agreed 👍

[slackpad]

Looking at https://golang.org/src/net/dnsclient_unix.go#L162, Go's built-in DNS client already does the right thing and asks over TCP if the UDP response is truncated. I'll do a little extra sanity checking but I think we might be done here! Adding SRV support would be a nice addition.

[slackpad]

Confirmed the behavior, if you set up the Consul agent to report truncation properly then the Go DNS client will use TCP and pull the full list:

{
        "dns_config": {
                "enable_truncate": true
        }
}

This kind of sucks though because we only really want this behavior for joining, it's not good in general to turn on truncation, especially with Consul's DNS where you only care about the first few because they are shuffled.

[slackpad]

Got a PR up for review. I decided against doing a SRV lookup because there's some ambiguity about port numbers because we have multiple gossip pools so this could lead to some confusion about which one we want to connect to. This keeps the current behavior of using the port if supplied, but otherwise using the configured port.