Implement Incremental crawling

[PROxZIMA]

Is your feature request related to a problem? Please describe.

Incremental Crawler visits a specific site multiple times in a specific interval of time. The main focus of an incremental crawl is to keep on updating the hyperlink list of a site whose content is being changed frequently. At every iteration, after the crawler visits such a site, the site’s “freshness” keeps on decreasing as time passes. Hence in the next iteration, the crawler updates the hyperlinks, resetting the freshness.

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