Heavy handling of fallbacks

Biggest overhead I've found (from NewRelic) is the hook_language_fallback_candidates_alter.
The only module that implements it is this one, so I've checked more what it is doing...

There are some issues that I was able to find.
This alter hook is triggered and it is generating the list every time when an entity needs to be translated. The thing is the set of fallbacks are generated from configuration every time (per entity).

In reality all of that is changing per entity bundle (not entity instance). In reality all of that can be cached, so in heavy translation scenarios we get, this list can be preserved in the long run. Maybe a persistent second level cache could help as well.

I think I've found the issue in my scenario - we have around 100 000 iterations related to translations of and to have heavy processing over them and prevent out of memory errors the static cache for the bundle is disabled during that list processing.

I suspect this code path is somewhat protected from entity static cache, but once that is disabled, the overhead is exposed (after some 100k iterations)..

I have a PoC patch that will be posted in near future for review and further discussions.

Here is the promised patch...

There is a new utility method on the controller that moves the alter code there.
There is a in-property cache for the values that will cache all of the things there per bundle, so we skip executions per entity.
Change in the interface, as there is a new method added.

I will try to have benchmarks available at near future as well, so we can see the performance difference this will make in 10k+ iterations.

I've managed to force the hook to be executed on a node with this snippet.

$nodeStorage = \Drupal::entityTypeManager()->getStorage('node');
$entityType = $nodeStorage->getEntityType();
$entityType->set('static_cache', FALSE);

/* @var $node \Drupal\node\Entity\Node */
$node = $nodeStorage->load(MY_NODE_ID);
$viewBuilder = \Drupal::entityTypeManager()->getViewBuilder('node');
foreach (range(1, 100) as $iteration) {
  $build = $viewBuilder->view($node, 'default', 'da');
  $viewBuilder->resetCache([$node]);
}

Time was aggregated with microtime() in the alter hook's code.
I am seeing a 40-50% speed-up on my simple local setup with 3 languages only.

Here is a patch with the discussed changes.