tracex: prepare HTTP code for future refactoring (#778)

The main issue I see inside tracex at the moment is that we
construct the HTTP measurement from separate events.

This is fragile because we cannot be sure that these events
belong to the same round trip. (Currently, they _are_ part
of the same round trip, but this is a fragile assumption and
it would be much more robust to dispose of it.)

To prepare for emitting a single event, it's imperative to
have two distinct fields for HTTP request and response headers,
which is the main contribution in this commit.

Then, we have a bunch of smaller changes including:

1. correctly naming 'response' the DNS response (instead of 'reply')

2. ensure we always use pointer receivers

Reference issue: https://github.com/ooni/probe/issues/2121

internal/engine/netx/tracex/archival.go

@@ -153,16 +153,16 @@ func newRequestList(begin time.Time, events []Event) []RequestEntry {
 		case *EventHTTPRequestMetadata:
 			entry.Request.Headers = make(map[string]MaybeBinaryValue)
 			httpAddHeaders(
-				ev.HTTPHeaders, &entry.Request.HeadersList, &entry.Request.Headers)
+				ev.HTTPRequestHeaders, &entry.Request.HeadersList, &entry.Request.Headers)
 			entry.Request.Method = ev.HTTPMethod
 			entry.Request.URL = ev.HTTPURL
 			entry.Request.Transport = ev.Transport
 		case *EventHTTPResponseMetadata:
 			entry.Response.Headers = make(map[string]MaybeBinaryValue)
 			httpAddHeaders(
-				ev.HTTPHeaders, &entry.Response.HeadersList, &entry.Response.Headers)
+				ev.HTTPResponseHeaders, &entry.Response.HeadersList, &entry.Response.Headers)
 			entry.Response.Code = int64(ev.HTTPStatusCode)
-			entry.Response.Locations = ev.HTTPHeaders.Values("Location")
+			entry.Response.Locations = ev.HTTPResponseHeaders.Values("Location")
 		case *EventHTTPResponseBodySnapshot:
 			entry.Response.Body.Value = string(ev.Data)
 			entry.Response.BodyIsTruncated = ev.DataIsTruncated