HADOOP-3366. Stall the shuffle while in-memory merge is in progress.

Files:
modified: /hadoop/core/trunk/src/java/org/apache/hadoop/mapred/RamManager.java (try)
modified: /hadoop/core/trunk/src/java/org/apache/hadoop/mapred/ReduceTask.java (try)
modified: /hadoop/core/trunk/src/java/org/apache/hadoop/mapred/MRConstants.java (try)
modified: /hadoop/core/trunk/CHANGES.txt (try)