only load unbroadcast tx if it exists in loaded mempool

Summary:
The unbroadcast_txids set only contains the txid. If for some reason a tx gets rejected when loading the mempool from the database, there is no point in loading the corresponding txid in the unbroadcast set, as we don't have the tx any longer for broadcasting.

This is a partial backport of core#18044 and core#19879
https://github.com/bitcoin/bitcoin/pull/18044/commits/c7eb6b4f1fe5bd76388a023529977674534334a7
https://github.com/bitcoin/bitcoin/pull/19879/commits/cb79b9dbf4cd06e17c8c65b36bf15c3ea2641de4

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11764