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/http.go

@@ -19,9 +19,9 @@ type CtrlHTTPResponse = webconnectivity.ControlHTTPRequestResult
 
 // HTTPConfig configures the HTTP check.
 type HTTPConfig struct {
-	Client            model.HTTPClient
 	Headers           map[string][]string
 	MaxAcceptableBody int64
+	NewClient         func() model.HTTPClient
 	Out               chan CtrlHTTPResponse
 	URL               string
 	Wg                *sync.WaitGroup
@@ -50,7 +50,9 @@ func HTTPDo(ctx context.Context, config *HTTPConfig) {
 			}
 		}
 	}
-	resp, err := config.Client.Do(req)
+	clnt := config.NewClient()
+	defer clnt.CloseIdleConnections()
+	resp, err := clnt.Do(req)
 	if err != nil {
 		config.Out <- CtrlHTTPResponse{ // fix: emit -1 like old test helper does
 			BodyLength: -1,