Convert comments to thread safety annotations

Summary: This is a backport of Core PR14772

Test Plan:

cmake -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ ..
ninja

Check that there are no thread safety analysis warnings.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4085