code: public void restartable(int restartableId, Func0<Subscription> factory) {
          restartables.put(restartableId, factory);
          if (requested.contains(restartableId))
              start(restartableId);
      }
docs: A restartable is any RxJava observable that can be started (subscribed) and
  should be automatically restarted (re-subscribed) after a process restart if
  it was still subscribed at the moment of saving presenter's state.
  
  Registers a factory. Re-subscribes the restartable after the process restart.
  
  @param restartableId id of the restartable
  @param factory       factory of the restartable