fix(measurex): use same keys of the OONI data format (#572)

This change should simplify the pipeline's job.

Reference issue: https://github.com/ooni/probe/issues/1817.

I previously dismissed this possibility, but now it seems clear it
is simpler to have a very tabular data format internally and to
convert such a format to OONI's data format when serializing.

The OONI data format is what the pipeline expects, but processing
is easier with a more linear/tabular format.

internal/tutorial/measurex/chapter01/README.md

@@ -167,8 +167,12 @@ format, in the remainder of this program we're
 going to serialize the `Measurement` to JSON and
 print it to the standard output.
 
+Rather than serializing the raw `Measurement` struct,
+we first convert it to the "archival" format. This is the
+data format specified at [ooni/spec](https://github.com/ooni/spec/tree/master/data-formats).
+
 ```Go
-	data, err := json.Marshal(m)
+	data, err := json.Marshal(measurex.NewArchivalDNSMeasurement(m))
 	runtimex.PanicOnError(err, "json.Marshal failed")
 	fmt.Printf("%s\n", string(data))
 ```
@@ -194,13 +198,14 @@ go run -race ./internal/tutorial/measurex/chapter01 | jq
 ```
 
 Where `jq` is being used to make the output more presentable.
+
 If you do that you obtain some logging messages, which are out of
 the scope of this tutorial, and the following JSON:
 
 ```JSON
 {
   "domain": "example.com",
-  "lookup_host": [
+  "queries": [
     {
       "answers": [
         {
@@ -237,6 +242,9 @@ the scope of this tutorial, and the following JSON:
 }
 ```
 
+This JSON [implements the df-002-dnst](https://github.com/ooni/spec/blob/master/data-formats/df-002-dnst.md)
+OONI data format.
+
 You see that we have two messages here. OONI splits a DNS
 resolution performed using the system resolver into two "fake"
 DNS resolutions for A and AAAA. (Under the hood, this is
@@ -266,7 +274,7 @@ This is the output JSON:
 ```JSON
 {
   "domain": "antani.ooni.org",
-  "lookup_host": [
+  "queries": [
     {
       "answers": null,
       "engine": "system",
@@ -327,7 +335,7 @@ To get this JSON:
 ```JSON
 {
   "domain": "example.com",
-  "lookup_host": [
+  "queries": [
     {
       "answers": null,
       "engine": "system",