code: public Observable<Boolean> shutdown() {
          for (;;) {
              FixedSchedulerPool curr = pool.get();
              if (curr == NONE) {
                  return Observable.just(true);
              }
              if (pool.compareAndSet(curr, NONE)) {
                  curr.shutdown();
                  return Observable.just(true);
              }
          }
      }
docs: TODO when depending on RxJava 1.0.15+, this would be an @Override of shutdown()