What's new in URLFrontier 2.2

What's Changed

• ShardedRocksDBService by @jnioche in #56
• RocksDB backend - faster restarts, implements #54 by @jnioche in #60
• DeleteCrawl - remove default value and need explicit crawlID
• Bugfix ID not returned by AbstractFrontierService
• PutURLs uses crawlid and URL as id for the URLInfo
• Getting a stack dump when closing RocksDB at the end of the service #17

Full Changelog: 2.1...2.2