- Refactors sending queue worker by breaking it into smaller tasks

- Adds arrayUnique method to Helpers for multidimensional arrays

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

@@ -0,0 +1,41 @@
+<?php
+namespace MailPoet\Cron\Workers\SendingQueue\Tasks;
+
+if(!defined('ABSPATH')) exit;
+
+class Subscribers {
+  static function get(array $queue) {
+    $subscribers = unserialize($queue['subscribers']);
+    if(empty($subscribers['processed'])) {
+      $subscribers['processed'] = array();
+    }
+    if(empty($subscribers['failed'])) {
+      $subscribers['failed'] = array();
+    }
+    return $subscribers;
+  }
+
+  static function updateToProcessList(
+    array $existing_subscribers_ids,
+    array $subscribers_to_process_ids,
+    array $queue_subscribers_to_process_ids
+  ) {
+    // compare existing subscriber to the ones that queued for processing
+    $subscibers_to_exclude = array_diff(
+      $subscribers_to_process_ids,
+      $existing_subscribers_ids
+    );
+    // remove nonexistent subscribers from the processing list
+    return array_diff(
+      $queue_subscribers_to_process_ids,
+      $subscibers_to_exclude
+    );
+  }
+
+  static function updateFailedList(array $subscribers, array $failed_subscribers) {
+    return array_merge(
+      $subscribers,
+      $failed_subscribers
+    );
+  }
+}