Was inadvertently passing null collections to Collection.addAll.
Added an additional XTraceContext.getThreadContext method to XTrace to
support the semantics we're looking for here.
Conflicts:

	hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/event/AsyncDispatcher.java
	hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-nodemanager/src/main/java/org/apache/hadoop/yarn/server/nodemanager/containermanager/localizer/ResourceLocalizationService.java