[datafeed] set delete-entities=yes is now safer, by checking each entity actually seems deleted. Closes #2165381 Parsers now have to implement the `is_deleted(extid, etype, eid)` method that should return a boolean telling if the given entity exists in the external source or not.

# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
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"""%(distname)s entity's classes"""