Fix -Wthread-safety-analysis warnings

Summary: The locks were missing from D4186.

Test Plan:
With Clang as a compiler:

cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug
ninja check

Ensure there is no -Wthread-safety-analysis warning in the build
output, and all the tests are passing.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4282