[atlas] DNS and recursion

API v1 had a field recursion_desired when performing DNS
measurements. If I remember correctly (but I cannot find the reference
right now), it worked only when querying a specific name server, and
not when using the probe's own resolver, to avoid cache snooping.

API v2 has instead a set_rd_bit. Documentation
<https://atlas.ripe.net/docs/api/v2/reference/#!/measurements/Measurement_List_POST>
does not indicate its default value.

Anyway, my experience with the API is that the RD (Recursion Desired)
bit is always set, whether with 'set_rd_bit': False or with
'set_rd_bit': True, and independant of use_probe_resolver.

I can understand this choice for privacy reasons (RFC 7626, section
2.3). But, in that case, the documentation should be fixed.

Anything I missed?

(Here, a request by a probe, the + after the Query ID - here, 52379 -
means the RD bit is set

12:22:23.170304 IP (tos 0x0, ttl 64, id 2287, offset 0, flags [DF], proto UDP (17), length 65)
    192.168.2.8.45955 > 212.27.40.240.53: [udp sum ok] 52379+ AAAA? uucp.bortzmeyer.org. (37)