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

(cherry picked from commit 3d67da502aa21d314d672f8b465d5415d77b5df0)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/BlockReaderLocal.java
HDFS-6934. Revert files accidentally committed.

(cherry picked from commit 5b1dfe78b8b06335bed0bcb83f12bb936d4c021b)
(cherry picked from commit 5dbd27f8b4923daea6e70b0bfe5cd3063a5f1f57)