Lock cs_main when calling the block finalized validation interface

Summary: For now there is a single subscriber (chronik) so there is no race, but this is a brittle design. Let's take the lock properly to avoid introducing issues in the future.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16801