CronJobs: Implement a task queue

[Mistralys]

Resource-intensive tasks should be allowed to be processed asynchronously. They are appended to the central task queue, and processed separately when possible.

Several ways to run pending tasks:

• Via a dedicated API
• Via Cron Jobs

Important things to keep in mind:

• Tasks must be prioritizable.
• Running tasks must be locked to avoid processing them multiple times.
• The lock must be cluster-compatible.