Merge -c 1471635 from branch-1 to branch-1.2 to fix MAPREDUCE-5166. Fix ConcurrentModificationException due to insufficient synchronization on updates to task Counters. Contributed by Sandy Ryza.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1.2@1471637 13f79535-47bb-0310-9956-ffa450edef68
Merge -c 1471644 from branch-1 to branch-1.2 to fix HADOOP-9458. Fix RPC.getProxy to ensure it uses retries for getProtocolVersion too. Contributed by Tsz Wo Sze.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1.2@1471646 13f79535-47bb-0310-9956-ffa450edef68