cubicweb #2165381 datafeed: be smarter with deletion [validation pending]

currently, our datafeed source have "delete-entities=no" option since it may be harmful. This could be made much safer by calling a parser method that would check an entity seems actually deleted (for instance, check its url leads to a 404)

done in3.15.0
load left0.000
closed by#981f6e487788 [datafeed] set delete-entities=yes is now safer, by checking each entity actually seems deleted. Closes #2165381