[WIP] Feature/search delete from index

Search deindexing on node delete and delete event from content mapper

- Added `loadShallowStructureForNode` method to ContentMapper
- Send out event for each deleted node and provide a Shallow structure to the listening events (in addition
  to the PHPCR node being deleted)

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | [yes|no => <WHY?>]
| Fixed tickets | [<WHICH?>|none]
| BC Breaks     | NO
| Doc           | NONE

Structure is lazy loaded, as we can optimize if nothing needs the structure.