Ensure that the unbroadcast set will still be meaningful if the node is
restarted.
This concludes backport of Core [[https://github.com/bitcoin/bitcoin/pull/18807 | PR18807]] [7/7]
https://github.com/bitcoin/bitcoin/pull/18038/commits/50fc4df6c4e8a84bdda13ade7bed7a2131796f00
Depends on D9011