Use MessagePack for DuplicateJobs cookies
This MR improves upon !100851 (merged). In !100851 (comment 1138373458) we had a discussion about the downsides of using APPEND. The conclusion was that we could fix this later. Having reflected on this, I think it is better to address this issue now so that it cannot surprise us in the future.
This MR changes the cookie format to MessagePack, with server side updates. This will keep the cookies small.