cover DisconnectBlock with lock annotation

Summary:
CoinsTip() access requires cs_main and therefore so should this function.

This is a backport of core#23661

Test Plan:
With clang + DEBUG
ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12456