Make $app['db.orm.cache'] return a new instance

[itavero]

Wouldn't it be better to change line 61 of DoctrineORMServiceProvider.php from this:

    'cache'    => new ArrayCache,

to this?

    'cache'    => function(){ return new ArrayCache(); },

That way the ArrayCache is only initialized once it's assigned to the Configuration (and it isn't when the user decides to use a different cache driver). It also ensures that the different aspects that can be cached (Query, Metadata and Result) aren't using the same cache, which is a good thing, right?

If we would want to share the same instance, we could also change it to $app->share( /* function above */ ).

I also suggest putting this approach in the README.md, as the recommended way to set a cache driver in the configuration.

I'd love to hear your opinion on all of this.