fix(oohelperd): use throw-away HTTPClient, Dialer, Resolver (#833)

This diff modifies the implementation of oohelperd in the master branch
to always use throw-away HTTPClient, Dialer, and Resolver.

The rationale of this change is to ensure we're not hitting limits of the
HTTPClient regarding the max number of connections per host.

This issue is described at https://github.com/ooni/probe/issues/2182.

While there, it feels more correct to use throw-away Dialer and Resolver.

We have a different patch for the release/3.15 branch because of
netx-related refactorings: https://github.com/ooni/probe-cli/pull/832.

internal/cmd/oohelperd/internal/webconnectivity/dns.go

@@ -19,16 +19,18 @@ type CtrlDNSResult = webconnectivity.ControlDNSResult
 
 // DNSConfig configures the DNS check.
 type DNSConfig struct {
-	Domain   string
-	Out      chan CtrlDNSResult
-	Resolver model.Resolver
-	Wg       *sync.WaitGroup
+	Domain      string
+	NewResolver func() model.Resolver
+	Out         chan CtrlDNSResult
+	Wg          *sync.WaitGroup
 }
 
 // DNSDo performs the DNS check.
 func DNSDo(ctx context.Context, config *DNSConfig) {
 	defer config.Wg.Done()
-	addrs, err := config.Resolver.LookupHost(ctx, config.Domain)
+	reso := config.NewResolver()
+	defer reso.CloseIdleConnections()
+	addrs, err := reso.LookupHost(ctx, config.Domain)
 	if addrs == nil {
 		addrs = []string{} // fix: the old test helper did that
 	}