Fix memory exhaustion when recreating duplicates-index (#512)

Author: natted

src/DuplicatesIndex/DefaultMariaDbDuplicatesIndex.php

@@ -117,6 +117,8 @@ public function recreateIndex()
 
                 $this->updateDuplicateIndexForCustomer($customer, true);
             }
+
+            \Pimcore::collectGarbage();
         }
     }