txmempool: add thread safety annotations

Summary:
Add missing thread safety guards to CTxMempool members.

This is a backport of core#22003

Test Plan:
With clang and debug:
ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11307