[3088] Added DNS request construction to d2::NameRemoveTransaction

Added methods for constructing all three types of DNS update requests
required by d2::NameRemoveTransaction to complete the implementation of its
state machine.  Also refactored some unit test code into nc_test_utils.h
and .cc. Renamed request verification functions in nc_test_utils to match
the build request function names.
[3088] Corrected commentary RFC references.

d2/nc_add.cc contained some inaccurate RFC references.
[3088] Addressed review changes.

Changes are largely clean up and commentary.
[3088] Minor review fixes.

Fixed two missed minor cleanups from the initial review.