Declare visibility for class properties in DatabaseOracle.php

* Remove unused $lastResult and $cursor.
* Rename private $ignore_DUP_VAL_ON_INDEX to $ignoreDupValOnIndex.
* Update/add some documentation.

Change-Id: Ie5d82836fe4907337e50f10cc1654d5ddf08f270
Merge "Declare visibility for class properties in DatabaseOracle.php"