leveldb: specialize StorageTxn in Cursor

Use exact type MojDbLevelTableTxn for reference in MojDbLevelCursor

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: I9eecf69ad211d1a8b8685ff8e2feb2a873caaec7
Reviewed-on: https://g2g.palm.com/2236
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>