Updated CHANGES.txt to reflect hadoop-2.6.0 release dates.

(cherry picked from commit 770cc144425b6188d9b93178c77a35cd830a60df)
YARN-2834. Fixed ResourceManager to ignore token-renewal failures on recovery consistent with the (somewhat incorrect) behaviour in the non-recovery case. Contributed by Jian He.
Fixed a minor import issue in the test during cherry-pick from trunk.

(cherry picked from commit e76faebc9589654e83c8244ef9aff88391e56b80)

Conflicts:
	hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/test/java/org/apache/hadoop/yarn/server/resourcemanager/TestWorkPreservingRMRestart.java
Release Notes for hadoop-2.6.0.
Preparing to release hadoop-2.6.0: Set version in branch-2.6 to 2.6.1-SNAPSHOT.