code: public boolean stopStandby() {
      for (int k = 0; k < mNumOfMasters; k++) {
        if (!mMasters.get(k).isServing()) {
          try {
            LOG.info("master {} is a standby. stopping it...", k);
            mMasters.get(k).stop();
            LOG.info("master {} stopped.", k);
          } catch (Exception e) {
            LOG.error(e.getMessage(), e);
            return false;
          }
          return true;
        }
      }
      return false;
    }
docs: Iterates over the masters in the order of master creation, stops the first standby master.
  
  @return true if a standby master is successfully stopped, otherwise, false