YARN-2977. Fixed intermittent TestNMClient failure. (Contributed by Junping Du)
CHANGES.txt: add YARN-2977
YARN-2975. FSLeafQueue app lists are accessed without required locks. (kasha)

(cherry picked from commit 24ee9e3431d27811530ffa01d8d241133fd643fe)
CHANGES.txt: move YARN-2977 from 2.6.1 to 2.7.0
Merge remote-tracking branch 'remotes/origin/branch-2' into branch-2