[Foundation] Dispose method freeing managed resource

Following the `Dispose` standard, one shouldn't need to dispose `CancellationTokenSource` in the `InflightData` class here -
```
protected virtual void Dispose (bool disposing)
{
if (disposing) {
if (CancellationTokenSource != null) {
CancellationTokenSource.Dispose ();
CancellationTokenSource = null;
}
}
}
```