Moved the requirement for memory visibility of a Reducers internal state to the framework by piggybacking it to an internal field
fixed checkstyle
Changed position for the happens-before
Update MR-Introduction.md

Fixed documentation notice on version
Update Reducer.java

Fixed Javadoc as well
Merge pull request #3868 from noctarius/mapreduce-issue-3866-maintenance

Moved the requirement for memory visibility of a Reducers internal state