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.
This commit moves the TH structs and definitions to model. We don't want
oohelperd to depend on web_connectivity@v0.4.
Part of https://github.com/ooni/probe/issues/2240