[ldapfeed / dataimport] port ldapfeed parser to dataimport API
This makes the code easier to understand and will allow to deprecate the old
multi-sources api, based on creation callback through repo.extid2eid.
Currently with this patch, modification dates are not checked, hence
entities will be systematically updated.
We run the import with no hooks, because RQLObjectStore can only add
entities to the system source.