Implement attemptMemoryReclaim() to reduce memory usage

[ChristophB]

When an Entity is loaded from the DB, it is stored in Drupals cache.
The cache may exceed the allowed maximum memory.

The following function should reclaim memory of entities, which were loaded but are not required anymore.

function attemptMemoryReclaim() {
    // First, try resetting Drupal's static storage - this frequently releases plenty of memory to continue.
    drupal_static_reset();
  
    // Entity storage can blow up with caches so clear them out.
    $manager = \Drupal::entityManager();
    foreach ($manager->getDefinitions() as $id => $definition) {
        $manager->getStorage($id)->resetCache();
    }
    return memory_get_usage();
}