svn merge -c 1579670 from trunk for HDFS-6129.  When a replica is not found for deletion, do not throw an exception.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1579671 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6089. Standby NN while transitioning to active throws a connection refused error when the prior active NN process is suspended. Contributed by Jing Zhao.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1579694 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6131. Move HDFSHighAvailabilityWithNFS.apt.vm and HDFSHighAvailabilityWithQJM.apt.vm from Yarn to HDFS. Contributed by Jing Zhao.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1579736 13f79535-47bb-0310-9956-ffa450edef68