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`);
- [x] transaction using COMMIT, ROLLBACK (in `Zend\Db\Adapter\Driver\IbmDb2\Connection.php`)

I'm testing the PR using unit tests and functional tests, with the usage of this tool: https://github.com/ezimuel/zf2dbtest.
I think with methods this short, there's no need for the extra line between statements. I can fix that on merge, though.