[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.

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile