VNet: Respect priority of ordered nameservers in /etc/resolv.conf

[ravicious]

When VNet receives a DNS query and it doesn't find a match for the given fqdn, it forwards the request to other nameservers used by the system.

teleport/lib/vnet/dns/dns.go

Lines 259 to 262 in 2b30fe5

	// Forward the message to each upstream nameserver concurrently, the first to answer wins.
	// Each goroutine will write a single error or a single response to the appropriate channel.
	// Each goroutine should quickly exit after the context is canceled.
	responses := make(chan []byte, len(upstreamNameservers))

Instead of racing them all, we should respect the priority of ordered nameservers as they're defined in /etc/resolv.conf and add a short delay for each one down the list.