Add TCP listener to the DNS filtering proxy

[ameshkov]

Intercept tcp://xxx:153, read the incoming DNS requests, reroute to the upstream, get the answer and write it back.

IMPORTANT: when you work with DNS over TCP, you need to encode the packet length in the first 2 bytes of every packet.

In order to debug return truncated responses to UDP requests, it will send the same request over TCP after that.

[ameshkov]

Waiting for newer dnsproxy version with blocking support.