Add missed thread safety annotations

Summary:
This is needed for upcoming commit "sync.h: Make runtime lock checks
require compile-time lock checks" to pass.

This is a backport of core#19668 [2/5]
https://github.com/bitcoin/bitcoin/pull/19668/commits/3ddc150857178bfb1c854c05bf9b526777876f56

Depends on D10161

Test Plan:
With TSAN:
ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10162