Moved the requirement for memory visibility of a Reducers internal state

... (maintenance)... to the framework by piggybacking it to an internal field

fixed #3866 
i like the changed sentence but I'm not sure the threadpool offers sufficient memory happens before guarantee and I don't would like to bet on this since it is not required and even so it might do now I wouldn't bet on that after possible internal changes of the executor implementations, WDYT?