Refactor DNS resources

[conectado]

This is needed to:

• Have an stable per-resource IP and have more stability with browsers or other applications that cache IPs for a long time
• Be able to apply filters on overlapping dns resources

The way to achieve this is:

• Have the client generate multiple unique ips per-fqdn in queries
• Pick the resource that will be used after we see traffic for those generated ips
• Have the gateway mangle traffic based on the associated fqdn for that ip
• Use source port nating to distinguish traffic in the gateway
• Do Ipv4-in-Ipv6 and Ipv6-in-Ipv4 to be able to work for ipv4/ipv6-only resources

[jamilbk]

refs #3369 -- we can probably close that when this is closed.