Replace RecursiveMutex m_cs_chainstate with Mutex, and rename it

Summary:

So apparently there is no recursion involved, so the m_cs_chainstate can be a non-recursive mutex.

Backport of core#24103.
Backport of core#24235.

Test Plan:
With Debug and Clang:

ninja all check

Run the TSAN build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12227