[ldaputils,ldapparser] correctly compute "deleted" users (closes #2542083)
* in ldaputils.py, removal of an unneeded method (checking existence
of an object in the ldap, but not using the configuration filters)
* in the ldapparser, memoize the (filtered) search so as to serve both
.process and .is_deleted
* add a non regression test for the 'user-filter' source config entry
+ deletion