HDFS-7128. Decommission slows way down when it gets towards the end. Contributed by Ming Ma.

(cherry picked from commit 80cb472620f0a7889e311ca32212a42b95c6794e)
(cherry picked from commit 7e949744b14e540cc2824eae8e01a3d99714d662)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeDescriptor.java