MAPREDUCE-4808. Refactor MapOutput and MergeManager to facilitate reuse by Shuffle implementations. (masokan via tucu)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1436936 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'trunk' of github.com:griddynamics/yhadoop-common into trunk-gd
Merge branch 'trunk-gd' of github.com:griddynamics/yhadoop-common into fb-HDFS-4311-trunk-gd