Add idIndex to Changelog Entries or PIT Group

Group-finding in some environments may use gidNumber=group.idIndex(). This won't work today when groups are deleted, because the idIndex is not available in PIT Groups. This leaves FullSync as the only way to clean up deleted groups. Further, FullSync can only delete groups if PSPNG is fully responsible for an ldap branch.

There are two possible solutions:
-Adding idIndex as a PIT Group attribute (probably too hard)
-Adding idIndex as a changelog-entry attribute