Replace multiple DB transactions with a single one

[phansys]

Subject

Replace multiple DB transactions with a single one.

I am targeting this branch, because these changes respect BC.

Closes #6280.

Changelog

### Changed
- Changed multiple calls to `ModelManagerInterface::find()` with `ModelManagerInterface::findBy()` in order to avoid multiple transactions.

[jordisala1991]

I think this wont be correct for composite keys

[phansys]

I agree, but at the beginning I thought this was occurring with the previous implementation. Checking EntityManager::find() I see that isn't the case.
I guess we must build a custom query for composite keys, since if we call findBy() for each key, we have no guarantee about the keys' consistency.

[jordisala1991]

What I dont know if it is better to delegate that to the persistence bundles, because I dont know if the odm works the same way. Maybe it is better to introduce the findMultiple you mentioned in the issue

[SonataCI]

Could you please rebase your PR and fix merge conflicts?

[phansys]

I'm currently using the same approach as CRUDController::batchAction(), which is relying on methods already present in ModelManagerInterface():

SonataAdminBundle/src/Controller/CRUDController.php

Line 535 in f57b683

$this->admin->getModelManager()->addIdentifiersToQuery($this->admin->getClass(), $query, $idx);

We could provide a shortcut ModelManagerInterface::findMultiple() based on this if we consider that as a requirement, but I'd like to know WDYT first.

[VincentLanglet]

@phansys I re-read again the PR.

Do we really need a ModelManagerInterface::findMultiple method ?

If not implemented you're using

$query = $this->modelManager->createQuery($this->class);
$this->modelManager->addIdentifiersToQuery($this->class, $query, $value);
$result = $this->modelManager->executeQuery($query);

We could always use this code instead of requiring a new method in the interface which can be fully implemented with others methods.

[phansys]

I think there is no need for a new method, but the same was also true in the first commit (I guess): d02a65a.
I agree with the proposal of not adding a new method and leveraging the API we already have (ModelManagerInterface::addIdentifiersToQuery()).
@sonata-project/contributors, please share any concerns about this in order to unblock this PR.
I'll try to find some time this weekend to revisit this.

[phansys]

Sorry @VincentLanglet, I had to fix a CS issue in the new test.
Please, review again.

[franmomu]

Nice

[VincentLanglet]

Thanks !