validation: Guard all chainstates with cs_main

Summary:
PR description:

This avoids a potential race-condition where a thread is reading the
ChainstateManager::m_active_chainstate pointer while another one is
writing to it. There is no portable guarantee that reading/writing the
pointer is thread-safe.

This is a backport of core#21025

Test Plan:
With debug and tsan:
ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11773