This change should simplify the pipeline's job.
Reference issue: https://github.com/ooni/probe/issues/1817.
I previously dismissed this possibility, but now it seems clear it
is simpler to have a very tabular data format internally and to
convert such a format to OONI's data format when serializing.
The OONI data format is what the pipeline expects, but processing
is easier with a more linear/tabular format.
