- Refactors sending queue worker by breaking it into smaller tasks

- Adds arrayUnique method to Helpers for multidimensional arrays

lib/Cron/Workers/SendingQueue/Tasks/Statistics.php

@@ -0,0 +1,30 @@
+<?php
+namespace MailPoet\Cron\Workers\SendingQueue\Tasks;
+
+use MailPoet\Models\StatisticsNewsletters;
+use MailPoet\Util\Helpers;
+
+if(!defined('ABSPATH')) exit;
+
+class Statistics {
+  static function updateBulkNewsletterStatistics(
+    array $processed_subscribers_ids, $newsletter_id, $queue_id
+  ) {
+    $newsletter_statistics = array();
+    foreach($processed_subscribers_ids as $subscriber_id) {
+      $newsletter_statistics[] = array(
+        $newsletter_id,
+        $subscriber_id,
+        $queue_id
+      );
+    }
+    $newsletter_statistics = Helpers::flattenArray($newsletter_statistics);
+    return StatisticsNewsletters::createMultiple($newsletter_statistics);
+  }
+
+  static function updateBulkNewsletterStatistics(
+    array $processed_subscribers_ids, $newsletter_id, $queue_id
+  ) {
+
+  }
+}