Add Elapsed property on the AllCrawlsCompleted event

[KeirLoire]

We would like to be able to retrieve the TimeSpan Elapsed property in the AllCrawlsCompleted event for the ParallelCrawlerEngine. Just like the other events like SiteCrawlCompleted.

For now, we're using a workaround by declaring a Stopwatch variable in the class scope which is started before crawler.StartAsync() is called. Then it is stopped inside AllCrawlsCompleted event to get the TimeSpan for the Elapsed time.