[backport#14193] validation: Add missing mempool locks

Summary:
https://github.com/bitcoin/bitcoin/pull/14193/commits/fabeb1f613653a8c1560e4a093a9b6b7a069b60b

Depends on D6537

Partial backport of Core PR14193

Test Plan:

cmake .. -DCMAKE_BUILD_TYPE=Debug
ninja check-all

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6539