HDFS-7228. Add an SSD policy into the default BlockStoragePolicySuite. Contributed by Jing Zhao.

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSOutputStream.java
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockStoragePolicySuite.java
	hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestBlockStoragePolicy.java
	hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/mover/TestStorageMover.java
HADOOP-11181. Generalized o.a.h.s.t.d.DelegationTokenManager to handle all sub-classes of AbstractDelegationTokenIdentifier. Contributed by Zhijie Shen.

(cherry picked from commit cdce88376a60918dfe2f3bcd82a7666d74992a19)
Merge remote-tracking branch 'remotes/origin/branch-2' into branch-2