Merge HDFS-5014. Process register commands with out holding BPOfferService lock. Contributed by Vinay.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1543862 13f79535-47bb-0310-9956-ffa450edef68
Merge r1543952 from trunk to branch-2 for YARN-1425. TestRMRestart fails because MockRM.waitForState(AttemptId) uses current attempt instead of the attempt passed as argument (Omkar Vinit Joshi via bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1543954 13f79535-47bb-0310-9956-ffa450edef68