code: public void finish() {
          if ( finSignal == null && finished )
              return;
          // signal finish of execution, so remoting can clean up callback id mappings
          // override if always single result or finish can be emitted by the remote method
          // note one can send FINSILENT to avoid the final message to be visible to receiver callback/spore
          cb.complete(null, null);
          finished = true;
      }
docs: to be called at remote side
  when using streaming to deliver multiple results, call this in order to signal no further
  results are expected.