fix: use tls.HandshakeContext

This has been a long-time TODO.

transport.go

@@ -76,39 +76,7 @@ func (t *Transport) handshake(
 	tlsConn *tls.Conn,
 	keyCh <-chan ci.PubKey,
 ) (sec.SecureConn, error) {
-	// There's no way to pass a context to tls.Conn.Handshake().
-	// See https://github.com/golang/go/issues/18482.
-	// Close the connection instead.
-	select {
-	case <-ctx.Done():
-		tlsConn.Close()
-	default:
-	}
-
-	done := make(chan struct{})
-	var wg sync.WaitGroup
-
-	// Ensure that we do not return before
-	// either being done or having a context
-	// cancellation.
-	defer wg.Wait()
-	defer close(done)
-
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		select {
-		case <-done:
-		case <-ctx.Done():
-			tlsConn.Close()
-		}
-	}()
-
-	if err := tlsConn.Handshake(); err != nil {
-		// if the context was canceled, return the context error
-		if ctxErr := ctx.Err(); ctxErr != nil {
-			return nil, ctxErr
-		}
+	if err := tlsConn.HandshakeContext(ctx); err != nil {
 		return nil, err
 	}