Notifier restart

### Is your feature request related to a problem? Please describe.
After I call `can.notifier.Notifier.stop` not a `Notifier` object, there is no obvious way how to restart the Notifier.
As far as I see, I need to create a new notifier, which leads to another problem - if a `Bus` is added to two Notifiers (even if one of them is stopped) then an exception is raised by `can.notifier._NotifierRegistry.register`.

### Describe the solution you'd like

I would like a `restart` method defined in `Notifier` class which would work exactly like I just created a new Notifier with the exactly the same settings as the existing one.

### Describe alternatives you've considered

- `can.notifier._NotifierRegistry.register` does not raise an exception if another Notifier is stopped