leveldb: Transaction and Cursors

- Implement database cursors for leveldb
- Implement database transaction for leveldb
- Make cursors to work via transaction

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <msditanov@luxoft.com>
Change-Id: I53872a55bc7fe5ad4162a7de7fbd6b409c3da73e
Reviewed-on: https://g2g.palm.com/2074
Reviewed-by: Build Verification
Reviewed-by: Nikolay Orliuk <norluk@luxoft.com>
Tested-by: Nikolay Orliuk <norluk@luxoft.com>
Reviewed-by: Maksym Sditanov <msditanov@luxoft.com>
Tested-by: Maksym Sditanov <msditanov@luxoft.com>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>