[backport#15932] rpc: Add lock annotations to block{,header}ToJSON

Summary:
https://github.com/bitcoin/bitcoin/pull/15932/commits/faea56400d5578023133cf4d1c761cdeb0c3e3da

Depends on D6297

Concludes backport of Core PR15932

Test Plan:

cmake .. -GNinja -DENABLE_SANITIZERS=thread
ninja check

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6300