This is the last bit of functionality we need before rewriting a
chunk of websteps to use netxlite.
To rewrite most of it, we still need to move over:
1. dnstransport code
2. errorsx code
With both done, netxlite is a good library for websteps as well
as for most other operations we perform outside of the experiments.
Part of https://github.com/ooni/probe/issues/1591
fe3c90479d