Single flush per "user-land" create

[kbond]

Currently, the following code calls $em->flush() twice. First, when creating the nested CategoryFactory and a second time after persisting the PostFactory.

PostFactory::new()->create([
    'category' => CategoryFactory::new(),
]);

It would be better if the nested CategoryFactory was just persisted, then after the PostFactory is persisted, a single flush occurs capturing all changes.

[kbond]

@voltel has noted significant performance issues when creating 1000's of factories. @wouterj, might be something to consider with #76?

One solution I thought of is to create a static Factory::delayFlush() method that accepts a callback where you create your factories/stories:

Factory::delayFlush(function() {
    PostFactory::new()->createMany(10000);

    // etc...
});

Before the callback is executed, flushing is disabled globally (new concept). Any factories created within the callback are just persisted. After the callback is executed, run flush and re-enable flushing globally.