Only start stopwatch when dataTableShowLogs && kDebugMode is true

[stan-at-work]

Now every build a stopwatch is created in debug and release mode.

If the properties dataTableShowLogs && kDebugMode are not true, it's created and started every build.

This is unnecessary.

Quick fix: Now

@override
Widget build(BuildContext context) {
var sw = Stopwatch();
sw.start();

...

sw.stop();

if (dataTableShowLogs && kDebugMode) {
  debugPrint('DataTable2 built: ${sw.elapsedMilliseconds}ms');
}

**How it could be**
```dart
    Stopwatch? sw;

  @override
  Widget build(BuildContext context) {
     if (dataTableShowLogs && kDebugMode){
     sw = Stopwatch();
     sw.start();
     }

...

    if (dataTableShowLogs && kDebugMode) {
      sw.stop();
      debugPrint('DataTable2 built: ${sw.elapsedMilliseconds}ms');
    }

[npateras]

And also pls add a property to disable the debug completely