Fixed invalid exception type thrown when calling tr.Cancel()

- return an FdbException with Code 'TransactionCancelled' if tr.Cancel() is called while CommitAsync() is running
- return a TaskCancelledException if the token passed to CommitAsync(..) is cancelled.
"Cancelled" vs "Canceled": The NeverEnding Story

- "Canceled" for public methods, to match the .NET Framework naming (TrySetCanceled / TaskCanceledException / TaskStatus.Canceled ...)
- "Cancelled" everywhere else (comments, messages, ...)