gtest_leveldb: added TestTxnIterator

- reversing (going forward, backward, forward, backward)
- reversing while walking through record that present in both txn and db
- check that we can get into tails return back to a records on the
  bounds
- ensure that all all records are visible and no other records returned
- seeks over data modified by txn, lower-bound, outside of key values
- corner cases for situation when we update txn while iterating over it

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: Idd767ed730162b34ca9386df0f8ce88842b8739e
Reviewed-on: https://g2g.palm.com/2249
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>