docs: AutoBuffer call(AutoBuffer ab) {
      int tnum = ab.getTask();
      RPC<?> t = ab._h2o.taskGet(tnum);
      assert t== null || t._tasknum == tnum;
      if( t != null ) t.response(ab); // Do the 2nd half of this task, includes ACKACK
      else ab.close();
      // Else forgotten task, but still must ACKACK
      return new AutoBuffer(ab._h2o).putTask(UDP.udp.ackack.ordinal(),tnum);
    }