Avoid locking CTxMemPool::cs recursively in simple cases

Summary:
Only trivial thread safety annotations and lock assertions added. No new locks.
No change in behavior, the lock is already held at call sites.

This is a backport of core#19854

Test Plan:
With TSAN:
ninja && ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10177