leveldb: fix reversing in TxnIterator

tests fixed:
- TestTxnIterator.reversing
- TestTxnIterator.tails
- TestTxnIterator.backAfterSeekOutside

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: I7438fd188cee99a5e6b50fbb16795c4de15f7cdc
Reviewed-on: https://g2g.palm.com/2254
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>