circumvention: add the tor experiment (#100)

* go.mod go.sum: update all non-probe-engine deps

For each line in the go.mod, run `go get -u -v $package` if the
line is not an indirect dependency and is not probe-engine.

Upgrading probe-engine is going to require the same spell that
is used in probe-engine to update psiphon.

* go get -v github.com/ooni/probe-engine@v0.5.0

This just pins to the latest probe-engine but we've not manually
pinned all the other dependencieds yet.

Take care of the trivial API changes in probe-engine as well, such
that we can have a working build after this commit.

* go.mod go.sum: pin to probe-engine dependencies

Basically: remove all indirect dependencies. Merge this go.mod with
the one of probe-engine, to pin dependencies. Run `go mod tidy`.

* circumvention: add basic implementation of tor

This needs to be polished further, of course. But at least we have
now added support for running tor in the circumvention group.

* Readme.md: document how to update dependencies

* go get -v github.com/ooni/probe-engine@fcc9ee0a7afb

* go get -v github.com/ooni/probe-engine@4d254f5b2

* nettests/tor.go: implement summary test keys

nettests/web_connectivity.go

@@ -10,7 +10,7 @@ func lookupURLs(ctl *Controller, limit int64) ([]string, map[int64]int64, error)
 	var urls []string
 	urlIDMap := make(map[int64]int64)
 	testlist, err := ctl.Ctx.Session.QueryTestListsURLs(&engine.TestListsURLsConfig{
-		Limit: int(limit),
+		Limit: limit,
 	})
 	if err != nil {
 		return nil, nil, err