Remove ServiceStartExecutable/Revertible and ServiceStopExecutable/Revertible. Rename SimpleService interface to Service interface.
Refactoring:

- extract StartingServiceTasks.StartServiceTask class  to non-inner StartServiceTask class;
- extract StoppingServiceTasks.StopServiceTask class to non-inner StopServiceTask class
- rename Service<Action>Task classes to <Action>ServiceTask;
- move creation of RemoveServiceTask into RemoveServiceTask to make it compliant with Stop/StartServiceTasks signature/structure.