refactor(ndt7): use netxlite rather than netx (#768)

This diff required us to move some code around, but no major
change actually happened, except better tests.

While there, I also slightly refactored ndt7's implementation and
removed the ProxyURL setting, which was actually unused.

See https://github.com/ooni/probe/issues/2121

internal/bytecounter/dialer.go

@@ -0,0 +1,52 @@
+package bytecounter
+
+//
+// model.Dialer wrappers
+//
+
+import (
+	"context"
+	"net"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// ContextAwareDialer is a model.Dialer that attempts to count bytes using
+// the MaybeWrapWithContextByteCounters function.
+//
+// Bug
+//
+// This implementation cannot properly account for the bytes that are sent by
+// persistent connections, because they stick to the counters set when the
+// connection was established. This typically means we miss the bytes sent and
+// received when submitting a measurement. Such bytes are specifically not
+// seen by the experiment specific byte counter.
+//
+// For this reason, this implementation may be heavily changed/removed
+// in the future (<- this message is now ~two years old, though).
+type ContextAwareDialer struct {
+	Dialer model.Dialer
+}
+
+// NewContextAwareDialer creates a new ContextAwareDialer.
+func NewContextAwareDialer(dialer model.Dialer) *ContextAwareDialer {
+	return &ContextAwareDialer{Dialer: dialer}
+}
+
+var _ model.Dialer = &ContextAwareDialer{}
+
+// DialContext implements Dialer.DialContext
+func (d *ContextAwareDialer) DialContext(
+	ctx context.Context, network, address string) (net.Conn, error) {
+	conn, err := d.Dialer.DialContext(ctx, network, address)
+	if err != nil {
+		return nil, err
+	}
+	conn = MaybeWrapWithContextByteCounters(ctx, conn)
+	return conn, nil
+}
+
+// CloseIdleConnections implements Dialer.CloseIdleConnections.
+func (d *ContextAwareDialer) CloseIdleConnections() {
+	d.Dialer.CloseIdleConnections()
+}