versionbits: Use dedicated lock instead of cs_main

Summary:
This is a backport of core#19438
https://github.com/bitcoin/bitcoin/pull/19438/commits/eccd736f3dc231ac0306ca763c3b72cf8247230a

GetNextBlockScriptFlags did not require to hold cs_main since D4100

Depends on D12218

Test Plan:
with clang and debug and tsan
ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12219