[backport#15932] rpc: Serialize in getblock without cs_main

Summary:
https://github.com/bitcoin/bitcoin/pull/15932/commits/fab00a5cb90053e9671627af6a35e57610a44e89

Depends on D6296

Partial backport of Core PR15932

Test Plan:

cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug
ninja check check-functional

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6297