Use wp-cron to trigger updates

This has a couple of advantages:

- No more locking (issues) on our side
- Integrates with all other WordPress cron plugins, e.g. see reclaim events in Cron-View
- Doesn't slow down user requests when update gets triggered
- No need to reinvent the wheel

Disadvantage:

- You might have to disable / re-enable Reclaim plugin to register with the scheduler
- Under some circumstances (failed hostname lookup on server, misconfiguration) wp-cron might not work. We could detect that and alarm the user. He has a more serious issue than just a non-updating reclaim.