Fix EZP-23681: Delayed indexing of subtree on move

Link: https://jira.ez.no/browse/EZP-23681

## Description
The only way to have a subtree indexed after a move is to use delayed indexing and a cronjob (for performance reason).

## Tests
Need to be done a bit more, still WIP

## TODO
- [x] Enjoy people's feedback on the approach
- [x] Sanity tests
- [x] fix unit test (or the code)
- [ ] move the $version signature modification to a separate commit ?
Q: Is it allowed by CS to do variable assignment inside a conditional like this? If yes, should it be doc-ed with a "Yes, I know what I'm doing" kind of comment? ;)