[atlas] Problems with DNS measurements and NSID

Maybe there's some confusion here, but this multiple response business 
is all related to the introduction of `resultset` in firmware 4610:

  https://atlas.ripe.net/docs/data_struct/#v4610_dns

When this firmware was introduced it became possible that instead of 
there being a single "result" property, a "resultset" property would 
exist containing a list of responses.  Rather than have the library act 
differently for older firmware results, I opted to have a consistent 
behaviour.  As to why this change was introduced in the first place, if 
there's interest I can ask Philip to respond to the list tomorrow when 
he's in since this is more of a question of probe behaviour rather than 
the parser library.