Check whether TCP support is needed/useful

[luc-x41]

The server does not return responses large enough to ever warrant falling back to TCP, but it is unknown whether there exist resolvers that default to querying via TCP. These would either time out and fall back, or time out altogether. UDP really is the standard for DNS so it seems unlikely, but given how many servers support TCP, such resolvers might exist and ideally we'd support all clients.

This would probably involve creating two threads to support both protocols. The TCP thread should prevent resource exhaustion due to excessive amounts of open connections.