[backport#14193] txpool: Make nTransactionsUpdated atomic

Summary:
https://github.com/bitcoin/bitcoin/pull/14193/commits/fa0c9dbf9156d64a4b9bff858da97825369a9134

Depends on D7439

Partial backport of Core PR14193

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6537