refactor(netxlite): add more functions to resolver (#455)

We would like to refactor the code so that a DoH resolver owns the
connections of its underlying HTTP client.

To do that, we need first to incorporate CloseIdleConnections
into the Resolver model. Then, we need to add the same function
to all netxlite types that wrap a Resolver type.

At the same time, we want the rest of the code for now to continue
with the simpler definition of a Resolver, now called ResolverLegacy.

We will eventually propagate this change to the rest of the tree
and simplify the way in which we manage Resolvers.

To make this possible, we introduce a new factory function that
adapts a ResolverLegacy to become a Resolver.

See https://github.com/ooni/probe/issues/1591.

internal/cmd/oohelperd/internal/websteps/explore.go

@@ -27,7 +27,7 @@ type Explorer interface {
 
 // DefaultExplorer is the default Explorer.
 type DefaultExplorer struct {
-	resolver netxlite.Resolver
+	resolver netxlite.ResolverLegacy
 }
 
 // Explore returns a list of round trips sorted so that the first

internal/cmd/oohelperd/internal/websteps/generate.go

@@ -24,7 +24,7 @@ type Generator interface {
 type DefaultGenerator struct {
 	dialer     netxlite.Dialer
 	quicDialer netxlite.QUICContextDialer
-	resolver   netxlite.Resolver
+	resolver   netxlite.ResolverLegacy
 	transport  http.RoundTripper
 }
 

internal/cmd/oohelperd/internal/websteps/initialchecks.go

@@ -31,7 +31,7 @@ type InitChecker interface {
 
 // DefaultInitChecker is the default InitChecker.
 type DefaultInitChecker struct {
-	resolver netxlite.Resolver
+	resolver netxlite.ResolverLegacy
 }
 
 // InitialChecks checks whether the URL is valid and whether the

internal/cmd/oohelperd/internal/websteps/measure.go

@@ -24,7 +24,7 @@ type Config struct {
 	checker   InitChecker
 	explorer  Explorer
 	generator Generator
-	resolver  netxlite.Resolver
+	resolver  netxlite.ResolverLegacy
 }
 
 // Measure performs the three consecutive steps of the testhelper algorithm:
@@ -87,10 +87,12 @@ func newDNSFailedResponse(err error, URL string) *ControlResponse {
 }
 
 // newResolver creates a new DNS resolver instance
-func newResolver() netxlite.Resolver {
+func newResolver() netxlite.ResolverLegacy {
 	childResolver, err := netx.NewDNSClient(netx.Config{Logger: log.Log}, "doh://google")
 	runtimex.PanicOnError(err, "NewDNSClient failed")
-	var r netxlite.Resolver = childResolver
-	r = &netxlite.ResolverIDNA{Resolver: r}
+	var r netxlite.ResolverLegacy = childResolver
+	r = &netxlite.ResolverIDNA{
+		Resolver: netxlite.NewResolverLegacyAdapter(r),
+	}
 	return r
 }