feat(engine): allow runner to return many measurements (#527)

This is required to implement websteps, which is currently tracked
by https://github.com/ooni/probe/issues/1733.

We introduce the concept of async runner. An async runner will
post measurements on a channel until it is done. When it is done,
it will close the channel to notify the reader about that.

This change causes sync experiments now to strictly return either
a non-nil measurement or a non-nil error.

While this is a pretty much obvious situation in golang, we had
some parts of the codebase that were not robust to this assumption
and attempted to submit a measurement after the measure call
returned an error.

Luckily, we had enough tests to catch this change in our assumption
and this is why there are extra docs and tests changes.

pkg/oonimkall/internal/tasks/runner.go

@@ -273,7 +273,11 @@ func (r *Runner) Run(ctx context.Context) {
 				Idx:     int64(idx),
 				Input:   input,
 			})
-			// fallthrough: we want to submit the report anyway
+			// Historical note: here we used to fallthrough but, since we have
+			// implemented async measurements, the case where there is an error
+			// and we also have a valid measurement cant't happen anymore. So,
+			// now the only valid strategy here is to continue.
+			continue
 		}
 		data, err := json.Marshal(m)
 		runtimex.PanicOnError(err, "measurement.MarshalJSON failed")