HDFS-7208. NN doesn't schedule replication when a DN storage fails.  Contributed by Ming Ma

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeDescriptor.java
HDFS-5089. When a LayoutVersion support SNAPSHOT, it must support FSIMAGE_NAME_OPTIMIZATION.