Queues are not triggered by CRON

Problem/Motivation

Executing mass-contact emails puts an item into the queue, but it never gets executed automatically by Drupal due to a missing key "cron" from the @QueueWorker annotation.

Steps to reproduce

1. Install a Drupal site with the module enabled.
2. Configure module, add a category (/admin/config/mass-contact/category/add), enable "Send message with Cron" and set "Maximum number of recipients before splitting up the email" to 1 (/admin/config/mass-contact/settings).
3. Add at least one user (/admin/people/create).
4. Send mail from /mass-contact
5. There should be one item at least in the mass_contact_queue_messages queue, check with drush queue-list.
6. Execute cron, either by running drush cron or from the UI (/admin/config/system/cron).
7. Check the queue again with drush queue-list, it should still show at least one item, and no emails are sent out. (If mailing is not set up, there should be an error message about that it couldn't send out any mails.)

Executing the queues directly sends out the emails, but not with cron.

Proposed resolution

Add missing "cron" key for the "@QueueWorker" annotations.