Are you sure that it is correct to call SaveAll from a TPL thread? That Seems Wrong.

If it isn't wrong, the Rx'y way to write this is:

```cs
documentChanged
    .SelectMany(_ => Observable.Start(() => SaveAll(), RxApp.TaskPoolScheduler))
    .Subscribe(_ => this.Log().Info("Saved all documents"), ex => this.Log().WarnException("Failed to save all documents", ex));
```