Review notification listeners

[inyono]

Finally got a reproduce case for the saving issues in staging:

1. Import staging database
2. Start API locally (and add it to definitions.local.php)
3. Try to save a new revision for /1855. Notice it takes quite a while. When logging, you'll notice the biggest factor are the notification listeners.

So we need to change that up. Some ideas:

• Atomic updates for notifications
• Combining multiple updates

604 deactivates the notification listeners for now. So to test it locally, you need to revert those changes.