Chapters stuck in "unprocessed" status.

[simjanos-dev]

There can be a rare case where a chapter gets stuck in "unprocessed status":

• Edit a chapter
• For some reason (like redis container goes down) horizon does not process the job inside the queue
• Horizon loses its state, for example LC version gets updated
• The chapter now is stuck in "unprocessed" status.

Need a method to check if a chapter has a job in the queue, or the whole queue has to be cleared. A chapter cannot be added to the processing queue twice, since that could lead to data loss.