BUGFIX: Make DefaultEventPublisherFactory return the same instance

Fixes `DefaultEventPublisherFactory::create()` to return the same
`EventPublisherInterface` instance for a given `$eventStoreIdentifier`.

This allows for injecting Event Publishers into 3rd party code with
some custom `Objects.yaml` configuration:

```yaml
Some\Vendor\Some\Class:
  properties:
    'eventPublisher':
      object:
        factoryObjectName: Neos\EventSourcing\EventPublisher\DefaultEventPublisherFactory
        arguments:
          1:
            value: 'Some-EventStore-Id'
```

Classes/EventPublisher/DefaultEventPublisherFactory.php

@@ -28,14 +28,24 @@ final class DefaultEventPublisherFactory implements EventPublisherFactoryInterfa
      */
     private $mappingProvider;
 
+    /**
+     * A list of all initialized Event Publisher instances, indexed by the "Event Store identifier"
+     *
+     * @var EventPublisherInterface[]
+     */
+    private $eventPublisherInstances;
+
     public function __construct(DefaultEventToListenerMappingProvider $mappingProvider)
     {
         $this->mappingProvider = $mappingProvider;
     }
 
     public function create(string $eventStoreIdentifier): EventPublisherInterface
     {
-        $mappings = $this->mappingProvider->getMappingsForEventStore($eventStoreIdentifier);
-        return DeferEventPublisher::forPublisher(new JobQueueEventPublisher($eventStoreIdentifier, $mappings));
+        if (!isset($this->eventPublisherInstances[$eventStoreIdentifier])) {
+            $mappings = $this->mappingProvider->getMappingsForEventStore($eventStoreIdentifier);
+            $this->eventPublisherInstances[$eventStoreIdentifier] = DeferEventPublisher::forPublisher(new JobQueueEventPublisher($eventStoreIdentifier, $mappings));
+        }
+        return $this->eventPublisherInstances[$eventStoreIdentifier];
     }
 }