[atlas] What is the meaning of 127.0.0.1 as probe resolver?

Hi,

While looking at the result of built-in DNS measurements that use the
probe resolvers, I noticed that a significant fraction of probes have
"127.0.0.1" as a resolver.  And the results are strange: performance is
really bad, for instance measurement 30002 gives a median resolution time
of 300 ms for these probes!

What could be the meaning of this?  It almost looks like a recursive
resolver with no cache at all is running on the probes themselves.

I was looking at this measurement: https://atlas.ripe.net/measurements/30002/

And here are some example probes that exhibit this "127.0.0.1" symptom:
6001, 6087, 6162, 6235, 6308.

Any reasonable explanation is welcome!

Thanks,
Baptiste
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