public void close() throws IOException {
        flushThreadPool.shutdown();
        fileUrlWriter.close();
        fileCursorWriter.close();
    }
Store urls and cursor in files so that a Spider can resume the status when shutdown.<br>

@author code4crafter@gmail.com <br>
@since 0.2.0
webmagic-extension/src/main/java/us/codecraft/webmagic/scheduler/FileCacheQueueScheduler.java