Merge #21025: validation: Guard chainman chainstates with cs_main

20677ffa22e93e7408daadbd15d433f1e42faa86 validation: Guard all chainstates with cs_main (Carl Dong)

Pull request 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 also done in way that mimics ::ChainstateActive(), so the
  transition from that function to this method is easy.

  More discussion:
  1. https://github.com/bitcoin/bitcoin/pull/20749#discussion_r559544027
  2. https://github.com/bitcoin/bitcoin/pull/19806#discussion_r561023961
  3. https://github.com/bitcoin/bitcoin/pull/19806#issuecomment-768946522
  4. https://github.com/bitcoin/bitcoin/pull/19806#issuecomment-768955695
  ```

  Basically this PR removes the loaded-but-unfired footgun, which:
  - Is multiplied (but still unshot) in the chainman deglobalization PRs (#20158)
  - Is shot in the test framework in the au.activate PR (#19806)

ACKs for top commit:
  jnewbery:
    code review ACK 20677ffa22e93e7408daadbd15d433f1e42faa86. I've verified by eye that neither of these members are accessed without cs_main.
  ryanofsky:
    Code review ACK 20677ffa22e93e7408daadbd15d433f1e42faa86. It is safer to have these new `GUARDED_BY` annotations and locks than not to have them, but in the longer run I think every `LOCK(cs_main)` added here and added earlier in f92dc6557a153b390a1ae1d0808ff7ed5d02c66e from #20749 should be removed and replaced with `EXCLUSIVE_LOCKS_REQUIRED(cs_main)` on the accessor methods instead. `cs_main` is a high level lock that should be explicitly acquired at a high level to prevent the chain state from changing. It shouldn't be acquired recursively in low-level methods just to read pointer values atomically.

Tree-SHA512: 68a3a46d79a407b774eab77e1d682a97e95f1672db0a5fcb877572e188bec09f3a7b47c5d0cc1f2769ea276896dcbe97cb35c861acf7d8e3e513e955dc773f89

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