[WIP] Fix for #6445, adding pagination and transaction to IBM DB2 for Zend\Db

This PR adds the following missing features to IBM DB2 adapter of `Zend\Db`:
- [X] pagination using the LIMIT and OFFSET sql statement (in `Zend\Db\Sql\Platform\IbmDb2\SelectDecorator.php`);
- [ ] transaction using COMMIT, ROLLBACK (in `Zend\Db\Adapter\Driver\IbmDb2\Connection.php`)

This is a Work In Progress, **please don't merge it**.
I'm refactoring the code a litte bit, I remove that part.