[datafeed] when checking for deletion using cwentity xml parser, use vid=xml to limit resources consumption

[repository] ldap-feed source. Closes #2086984 datafeed based source which copy a subtree of the ldap directory into the system database. Authentication still go through ldap though. Pros: * don't need temporary tables and such for multi-sources RQL queries execution * much more flexible to enhance / configure behaviour (you simply have to replace the parser) * run better when ldap isn't reachable Cons: * no more 'on the fly' discovery of users (though a user authenticating itself will be automatically added if it doesn't exist in the db yet) * synchronization may be heavy if there are a lot of users A new cw.server.ldaputils containing code in common between former ldapuser and new ldapfeed sources has been introduced. Also ldapuser source now uses url instead of custom host/protocol option so it looks like a datafeed source (could be improved).

[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.