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
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.