leveldb: always use EnvTxn as StorageTxn

- Fix MojDbQuota caused by reporting offsetQuota to a wrong StorageTxn
- Kick off TableTxn from StorageTxn hierarchy. When we use EnvTxn we
  shouldn't have any other StorageTxn.
- TODO in MojDbLevelTxn.h

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: Iee15dc89576d7fee89b6e15bf082ed3245631acd
Reviewed-on: https://g2g.palm.com/2237
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>