leveldb: wrapper for leveldb iterator

This MojDbLevelIterator will be used in future commits to iterate over
leveldb and transaction data in a uniform way.

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: I1198a6b89517c2b33dbe94febb66da65fb43a2ee
Reviewed-on: https://g2g.palm.com/2238
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>