Fix EZP-23116: Change SPI Content Search Handler to return ContentInfo

Does not affect API, idea is to avoid having to return the full objects for:
- faster search (less work by search, loading hitting persistence cache)
- make sure search performance stays realtive to load() performance as languages are added*

* Further work will improve this so the remaining performance impact of adding more languages
 is completely gone as long as long as language filters on api is cortrectly used.