HDFS-5164.  DeleteSnapshot should check if OperationCategory.WRITE is possible before taking write lock (contributed by Colin Patrick McCabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1520492 13f79535-47bb-0310-9956-ffa450edef68
HDS-5118. Provide testing support for DFSClient to drop RPC responses. Contributed by Jing Zhao.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1520637 13f79535-47bb-0310-9956-ffa450edef68
Merge remote-tracking branch: upstream/trunk