Add Core\Persistence\Readme.md for orientation
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.
Merge pull request #995 from ezsystems/search_performance

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