@Override
        public void onRemoval(RemovalNotification<String, LlapTokenLocalClient> notification) {
          if (notification.getValue() != null) {
            notification.getValue().close();
          }
        }
We'll keep signers per cluster around for some time, for reuse.
llap-client/src/java/org/apache/hadoop/hive/llap/coordinator/LlapCoordinator.java