DnsClient: Updated implementation to replace "PreferIPv6" feature with "IPv6Mode" feature which allows specifying additional option to enable IPv6 support without preferring it over IPv4. Updated RecursiveResolveAsync() to trigger NS resolution only on successful resolution of request to avoid attacks. Updated RecursiveResolveAsync() to implement new feature to mark NS record for name server as misconfigured to avoid using it for a while to mitigate various attacks.