[backport#15652] wallet: Update transactions with current mempool after load

Summary:
https://github.com/bitcoin/bitcoin/pull/15652/commits/2ebf650b2eb7a078ab60c8c4d5c726823686f549

Depends on D6246

Partial backport of Core PR15652

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6247