Zend\Log\Writer\Db via config throws exception

I use this config to create a logger:

```
'log' => array(
    'Application\Log' => array(
        'writers' => array(
            array(
                'name'     => 'db',
                'priority' => 1,
                'options'  => array(
                    'separator' => '_',
                    'column'    => array(),
                    'table'     => 'applicationlog',
                    'db'        => 'Zend\Db\Adapter\Adapter',
                ),
            ),
        ),
        #'exceptionhandler' => true,
        #'errorhandler'     => true,
    ),
),
```
With this config I can get the logger via the service manager:
```php
$serviceManager->get('Application\Log');
```

This results in the following exception:
> Zend\Log\Exception\InvalidArgumentException: You must pass a valid Zend\Db\Adapter\Adapter in [...]\vendor\zendframework\zendframework\library\Zend\Log\Writer\Db.php on line 74

`Zend\Log\Writer\Db` expects an instance of `Zend\Db\Adapter\Adapter` for the option key `db`.

Suggested fixes:
- If `db` is a string, use the service manager to get the class by the name provided in the `db` option key.
- If `db` is a closure, get the result of the closure.

When the string possibility and the closure possibility is checked and processed there can be checked if this results in a valid instance of `Zend\Db\Adapter\Adapter`.