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/tcpconnect.go

@@ -20,16 +20,18 @@ type TCPResultPair struct {
 
 // TCPConfig configures the TCP connect check.
 type TCPConfig struct {
-	Dialer   model.Dialer
-	Endpoint string
-	Out      chan TCPResultPair
-	Wg       *sync.WaitGroup
+	Endpoint  string
+	NewDialer func() model.Dialer
+	Out       chan TCPResultPair
+	Wg        *sync.WaitGroup
 }
 
 // TCPDo performs the TCP check.
 func TCPDo(ctx context.Context, config *TCPConfig) {
 	defer config.Wg.Done()
-	conn, err := config.Dialer.DialContext(ctx, "tcp", config.Endpoint)
+	dialer := config.NewDialer()
+	defer dialer.CloseIdleConnections()
+	conn, err := dialer.DialContext(ctx, "tcp", config.Endpoint)
 	if conn != nil {
 		conn.Close()
 	}