Add index based on subscriber_id to scheduled_task_subscribers

[MAILPOET-1382]

This is done to avoid full table scans when scheduling welcome emails
and checking whether or not an email has been sent.
There we have to query based on `subscriber_id`, but without having a
`task_id`, so MySQL cannot make use of the `(task_id, subscriber_id)`
primary key.

lib/Config/Migrator.php

@@ -131,7 +131,8 @@ class Migrator {
       'subscriber_id int(11) unsigned NOT NULL,',
       'processed int(1) NOT NULL,',
       'created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,',
-      'PRIMARY KEY  (task_id, subscriber_id)',
+      'PRIMARY KEY  (task_id, subscriber_id),',
+      'KEY subscriber_id (subscriber_id)'
     );
     return $this->sqlify(__FUNCTION__, $attributes);
   }