leveldb: take TxnIterator ownership to Cursor

- move ownership for TxnIterator from TableTxn to Cursor
- allow having TableTxn without TxnIterator
- allow multiple Cursors/TxnIterators per TableTxn

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: I0dde76ab18c637b6f2a260a66fb01685c52c7c06
Reviewed-on: https://g2g.palm.com/2204
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>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>
leveldb: fixed issue with updates in transaction

- Simplified getValue and getKey
- Advance both iterators when keys are the same. Otherwise we'll
  report back two records with same key.

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: I918269328416f5c63132951fc2a6cc3de558fb87
Reviewed-on: https://g2g.palm.com/2225
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>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>