While working on https://github.com/ooni/probe/issues/2237, I noticed
there's no enforced timeout for measurement tasks.
So, this diff introduces the following timeouts:
1. use a 4 seconds timeout for the DNS lookup;
2. use a 10 seconds timeout for TCP;
3. use a 15 seconds timeout for HTTP.
They are a bit stricter than what we have on the probe because the TH
should supposedly have better bandwidth and connectivity.
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.
