[mempool] Persist unbroadcast set to mempool.dat

Summary:
Ensure that the unbroadcast set will still be meaningful if the node is
restarted.

This concludes backport of Core PR18807 [7/7]
https://github.com/bitcoin/bitcoin/pull/18038/commits/50fc4df6c4e8a84bdda13ade7bed7a2131796f00
Depends on D9011

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9012