This ensures one can't instanciate a DNSResolver object by
mistake, but uses the singleton. A separate create static
function is added for cases where a new object is explicitely
needed.
Based on tewinget's update.
Make OpenAlias address format independent of existing DNS functions.
Add tests.
Test:
make debug-test
cd build/debug/tests/unit_tests
# test that regular DNS functions work, including IPv4 lookups.
# also test function that converts OpenAlias address format
make && ./unit_tests --gtest_filter=DNSResolver*
# test that OpenAlias addresses like donate@getmonero.org work from
# wallet tools
make && ./unit_tests --gtest_filter=AddressFromURL.Success
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.
