refactor(netx/dialer): hide implementation complexity (#372)

* refactor(netx/dialer): hide implementation complexity

This follows the blueprint of `module.Config` and `nodule.New`
described at https://github.com/ooni/probe/issues/1591.

* fix: ndt7 bug where we were not using the right resolver

* fix(legacy/netx): clarify irrelevant implementation change

* fix: improve comments

* fix(hhfm): do not use dialer.New b/c it breaks it

Unclear to me why this is happening. Still, improve upon the
previous situation by adding a timeout.

It does not seem a priority to look into this issue now.

internal/engine/legacy/netx/dialer.go

@@ -64,17 +64,19 @@ func maybeWithMeasurementRoot(
 // - dialer.Default
 //
 // If you have others needs, manually build the chain you need.
-func newDNSDialer(resolver dialer.Resolver) dialer.DNSDialer {
-	return dialer.DNSDialer{
-		Dialer: EmitterDialer{
-			Dialer: dialer.ErrorWrapperDialer{
-				Dialer: dialer.ByteCounterDialer{
-					Dialer: dialer.Default,
-				},
-			},
-		},
-		Resolver: resolver,
-	}
+func newDNSDialer(resolver dialer.Resolver) dialer.Dialer {
+	// Implementation note: we're wrapping the result of dialer.New
+	// on the outside, while previously we were puttting the
+	// EmitterDialer before the DNSDialer (see the above comment).
+	//
+	// Yet, this is fine because the only experiment which is
+	// using this code is tor, for which it doesn't matter.
+	//
+	// Also (and I am always scared to write this kind of
+	// comments), we should rewrite tor soon.
+	return &EmitterDialer{dialer.New(&dialer.Config{
+		ContextByteCounting: true,
+	}, resolver)}
 }
 
 // DialContext is like Dial but the context allows to interrupt a