[atlas] Invalid JSON because of DNS TXT records

The DNS, as we know, is a jungle. People put anything in their TXT records, even control characters. Atlas faithfully returns this content in the JSON files (see measurement #2004707). Then they are no longer legal JSON (RFC 7159, section 7) and crash the JSON libraries. Who should fix it? My first guess is that Atlas should not produce illegal JSON files and therefore should escape such texts.