public synchronized void shutdownNow() {
    if (pool == null || pool.isShutdown()) {
      return;
    }
    pool.shutdownNow();
  }
Checks if pool can be updated. If so, mark for update later.
@param conf configuration
hbase-server/src/main/java/org/apache/hadoop/hbase/master/cleaner/DirScanPool.java