make sure master restarts dsService if twill services are already running

Issue is this: If the master becomes leader, it checks whether twill services are running. 
- If not, it starts the twill services, and once they are running also starts the dsService
- I yes, it simply leaves them running and does nothing. But it must start the dsService. 
Since `dsService` is a guava service, once it is stopped, it cannot be started again (you'll get exception). You need a new instance of `DatasetService` (through `injector.getInstance()` and need to make sure that binding is not a singleton binding) in order to be able to start.