[3089] Addressed review comments.

Changed DNSClient ctor to require response param be
an empty pointer and clarified related commentary.
Added debug log statement to log transaction start.
Other minor cosmetics.
[3089] Added commentary and corrected doxygen errors

Added commentary to d2/tests/nc_test_utils.h::FauxServer
Corrected doxygen errors
[3089] Added ticket number to commentary

Added todo to dns_client.cc commentary regarding
trac# 3286 and the behavior isc::dns::Message::fromWire.