Werk #12167: Fixed acknowledgement expiry

Previously, the expiration mechanism for acknowledgements ignored the "persistent" flag, so the corresponding comments always vanished after expiration. Furthermore, the expiration of one acknowledgement didn't remove all other acknowledgements for the same host/service.

Both aspects were inconsistent with the behavior of the manual removal of acknowledgements, which was quite confusing and unintended. This has been fixed, so acknowledgement expiry is identical to manual removal at the given point in time.

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