HDFS-4516. Client crash after block allocation and NN switch before lease recovery for the same file can cause readers to fail forever. Contributed by Vinay.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1543829 13f79535-47bb-0310-9956-ffa450edef68
Merge remote-tracking branch: upstream/trunk