Closes https://github.com/ooni/probe/issues/2183
While there, avoid exposing nil values for optional fields of the
THResponse struct (i.e., "ip_info" and "tls_handshake").
While there, fix `measurexlite`'s `OperationLogger` test
and make it deterministic rather than racy.
In https://github.com/ooni/probe-cli/pull/832's initial diff, I
mentioned it would be cool to flatten oohelperd's hier.
I'm doing this now, and just for the master branch.
This diff is mostly a mechanical refactoring with very light
and apparently rather safe manual changes.
