HDFS-5024. Make DatanodeProtocol#commitBlockSynchronization idempotent. Contributed by Arpit Agarwal.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1506789 13f79535-47bb-0310-9956-ffa450edef68
Merge remote-tracking branch upstream/$:{upbranch}