DNSSEC is now implemented with the hardcoded key from unbound.
This will need to be not hardcoded in the future, but is okay for now.
Unit tests updated for DNSSEC (as well as for the fact that, contrary to
previous assumption, example.com does not have a static IP address).
Note: DNSResolver does not yet *use* DNSSEC, but rather this commit is
preparation for including DNSSEC validation. The function in
src/wallet/wallet2.cpp that uses DNSResolver still needs its parameters
updated accordingly.
ldns dependency was only still around for constants defined in ldns/rr.h,
but those constants are RFC specified DNS constants, and to reduce deps
have been replicated in dns_utils.h instead of including ldns/rr.h.