[mempool] sanity check that all unbroadcast txns are in mempool

Summary:

• before reattempting broadcast for unbroadcast txns, check they are in mempool and remove if not
• this protects from memory leaks and network spam just in case unbroadcast set (incorrectly) has extra txns
• check that tx is in mempool before adding to unbroadcast set to try to prevent this from happening

This is a backport of Core PR18895 [3/3]
https://github.com/bitcoin/bitcoin/pull/18895/commits/9d3f7eb9860254eb787ebe2734fd6a26bcf365c1
Depends on D9024

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9025