rpc: reduce LOCK(cs_main) scope in blockToJSON

Summary:
This is a partial backport of core#26308
https://github.com/bitcoin/bitcoin/pull/26308/commits/efd82aec8a2dd0fca8f2597c3f84cefe057d1243

Depends on D13054

Test Plan:
With Clang and DEBUG:
ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13055