Merge #17737: Add ChainstateManager, remove BlockManager global

c9017ce3bc27665594c9d80f395780d40755bb22 protect g_chainman with cs_main (James O'Beirne)
2b081c4568e8019886fdb0f2a57babc73d7487f7 test: add basic tests for ChainstateManager (James O'Beirne)
4ae29f5f0c5117032debb722d7049664fdceeae8 use ChainstateManager to initialize chainstate (James O'Beirne)
5b690f0aae21e7d46cbefe3f5be645842ac4ae3b refactor: move RewindBlockIndex to CChainState (James O'Beirne)
89cdf4d5692d396b8c7177b3918aa9dab07f9624 validation: introduce unused ChainstateManager (James O'Beirne)
8e2ecfe2496d8a015f3ee8723025a438feffbd28 validation: add CChainState.m_from_snapshot_blockhash (James O'Beirne)

Pull request description:

  This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11):

  Parent PR: #15606
  Issue: #15605
  Specification: https://github.com/jamesob/assumeutxo-docs/tree/master/proposal

  ---

  This changeset introduces `ChainstateManager`, which is responsible for creating and managing access to multiple chainstates. Until we allow chainstate creation from UTXO snapshots (next assumeutxo PR?) it's basically unnecessary, but it is a prerequisite for background IBD support.

  Changes are also made to the initialization process to make use of `g_chainman` and thus clear the way for multiple chainstates being loaded on startup.

  One immediate benefit of this change is that we no longer have the `g_blockman` global, but instead have the ChainstateManager inject a reference of its shared BlockManager into any chainstate it creates.

  Another immediate benefit is that uses of `ChainActive()` and `ChainstateActive()` are now covered by lock annotations. Because use of `g_chainman` is annotated to require cs_main, these two functions subsequently follow.

  Because of whitespace changes, this diff looks bigger than it is. E.g., 4813167d98 is most easily reviewed with
  ```sh
  git show --color-moved=dimmed_zebra -w 4813167d98
  ```

ACKs for top commit:
  MarcoFalke:
    re-ACK c9017ce3bc27665594c9d80f395780d40755bb22 📙
  fjahr:
    Code Review Re-ACK c9017ce3bc27665594c9d80f395780d40755bb22
  ariard:
    Code Review ACK c9017ce
  ryanofsky:
    Code review ACK c9017ce3bc27665594c9d80f395780d40755bb22. No changes since last review other than a straight rebase

Tree-SHA512: 3f250d0dc95d4bfd70852ef1e39e081a4a9b71a4453f276e6d474c2ae06ad6ae6a32b4173084fe499e1e9af72dd9007f4a8a375c63ce9ac472ffeaada41ab508

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