HDFS-6934. Move checksum computation off the hot path when writing to RAM disk. Contributed by Chris Nauroth.

(cherry picked from commit 463aec11718e47d4aabb86a7a539cb973460aae6)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSOutputStream.java