NUTCH-1588 Port NUTCH-1245 URL gone with 404 after db.fetch.interval.max stays db_unfetched in CrawlDb and is generated over and over again to 2.x

git-svn-id: https://svn.apache.org/repos/asf/nutch/branches/2.x@1538200 13f79535-47bb-0310-9956-ffa450edef68