Split leveldb classes to separated files

- Split leveldb classes to separated files
- Make correct includes inside leveldb classes
- Added #error directive for engine selection inside tests

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <msditanov@luxoft.com>

Change-Id: Ib5ea9f003c78cf8d0c451a8fc074d81a915f29ea