Use new set instead of add in BatchThreadPoolDescriptionResolver, and use array instead of list in ChainedContextHandle

batch subsystem ChainedContextHandle currently has the following constructor, which saves context handlers into a list. We should be able to skip the list creation.

        public ChainedContextHandle(final ContextHandle... contextHandles) {
            this.contextHandles = Arrays.asList(contextHandles);
        }

BatchThreadPoolDescriptionResolver currently adds certain elements into a new set. We should be able to just create a new set.