Refactor DnsServer

[mikee47]

This PR is just a refactor of the DnsServer class as it's a bit more complex than necessary. No additional functionality added, but a few extra bounds checks have been added.

• Move downcaseAndRemoveWwwPrefix, getDomainNameWithoutWwwPrefix and requestIncludesOnlyOneQuestion into private namespace (with appropriate parameter) - these do not need to be class members.

• Remove dnsHeader and buffer, these are only used locally.

• Move bulk of logic into separate processQuestion method.

• Add additional debug output, printing content of question/response packets.

• Fix potential issue with getDomainNameWithoutWwwPrefix if www. is contained within the domain name (unlikely, but possible)

• Don't allocate response buffer on stack - size is potentially unbounded. Instead, re-use the allocated buffer for the response, simpler as our answer is just appended to the received question.

Tested using nslookup with santizers active.