YARN-2830. Add backwords compatible ContainerId.newInstance constructor. Contributed by Jonathan Eagles.

(cherry picked from commit 43cd07b408c6613d2c9aa89203cfa3110d830538)
HADOOP-10786. Fix UGI#reloginFromKeytab on Java 8. Contributed by Stephen Chu.
HDFS-7383. DataNode.requestShortCircuitFdsForRead may throw NullPointerException. Contributed by Tsz Wo Nicholas Sze.
HADOOP-10786. Moved to hadoop-2.7.X.

(cherry picked from commit 14b87b70a8dfc03801dcf5f33caa7fd2cc589840)
HDFS-7383. Moved the jira from release 2.7.0 section to 2.6.0 in CHANGES.txt
HDFS-7383. Merged to branch-2.6 also.

(cherry picked from commit f62ec31739cc15097107655c6c8265b5d3625817)
Revert "HDFS-7383. Merged to branch-2.6 also."

This reverts commit 755af2768b2be1ded532e795db30ec0bfea03e66.