Fixed limit applying for Oracle queries.

In most DB limit applying on query done through concatenating query with limit string, like `LIMIT n OFFSET m`. Such case in Oracle done through wrapping whole query by special query. In all schemes, except `\yii\db\oci`, `QueryBuilder::buildLimit()` returns limit string for concatenating. `\yii\db\oci\QueryBuilder::buildLimit()` returns whole query with applied limit.
At first this brakes method behaviour. Also such behaviour does not work in `SqlDataProvider`.