Add Clang thread safety annotations for variables guarded by cs_db

Summary:
Backport of core PR13127:
https://github.com/bitcoin/bitcoin/pull/13127/files

Depends on D4886.

Test Plan:

cmake -GNinja .. -DENABLE_WERROR=ON
ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4888