Replace multiple subscriber update queries with one in sending loop

[MAILPOET-5178]

mailpoet/lib/Tasks/Subscribers.php

@@ -37,11 +37,13 @@ class Subscribers {
 
   public function updateProcessedSubscribers(array $processedSubscribers) {
     if (!empty($processedSubscribers)) {
-      $this->getSubscribers()
-        ->whereIn('subscriber_id', $processedSubscribers)
-        ->findResultSet()
-        ->set('processed', ScheduledTaskSubscriber::STATUS_PROCESSED)
-        ->save();
+      ScheduledTaskSubscriber::rawExecute(sprintf(
+        'UPDATE %1$s SET processed = %2$s WHERE task_id = %3$s AND subscriber_id IN (%4$s)',
+        ScheduledTaskSubscriber::$_table,
+        ScheduledTaskSubscriber::STATUS_PROCESSED,
+        $this->task->id,
+        join(', ', array_map('intval', $processedSubscribers))
+      ));
     }
     $this->checkCompleted();
   }