[backport#15365] wallet: Add lock annotation for mapAddressBook

Summary:
https://github.com/bitcoin/bitcoin/pull/15365/commits/faa46475d7d7dc60dcc80be34826cf98fb25dc95

Backport of Core PR15365

Test Plan:

cmake .. -GNinja -DENABLE_SANITIZERS=thread -DENABLE_WERROR=ON

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6282