leveldb: consider put/del effect on transaction

- assert that we never del data which current in-transaction-iterator
  points to
- handle insertion of new values in transaction that should affect
  further advances of TxnIterator

fixes tests:
- TestTxnIterator.jumpOffDeleted
- TestTxnIterator.jumpBackOffDeleted

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: I0b2140965b79e7d0aad45cf4fba0024729f8d37f
Reviewed-on: https://g2g.palm.com/2255
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>