code: public void start(long previousCrawlDuration) {
  		requestLogger.start(previousCrawlDuration);
  		resourceLogger.start(previousCrawlDuration);
  		transferredBytesLogger.start(previousCrawlDuration);
  		receivedURLsLogger.start(previousCrawlDuration);
  	}
docs: Starst all progress loggers.
  
  @param previousCrawlDuration the duration of the previous crawl, or zero for a new crawl.