Merge #17737: Add ChainstateManager, remove BlockManager global

c9017ce3bc protect g_chainman with cs_main (James O'Beirne)
2b081c4568 test: add basic tests for ChainstateManager (James O'Beirne)
4ae29f5f0c use ChainstateManager to initialize chainstate (James O'Beirne)
5b690f0aae refactor: move RewindBlockIndex to CChainState (James O'Beirne)
89cdf4d569 validation: introduce unused ChainstateManager (James O'Beirne)
8e2ecfe249 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 c9017ce3bc 📙
  fjahr:
    Code Review Re-ACK c9017ce3bc
  ariard:
    Code Review ACK c9017ce
  ryanofsky:
    Code review ACK c9017ce3bc. No changes since last review other than a straight rebase

Tree-SHA512: 3f250d0dc95d4bfd70852ef1e39e081a4a9b71a4453f276e6d474c2ae06ad6ae6a32b4173084fe499e1e9af72dd9007f4a8a375c63ce9ac472ffeaada41ab508

src/test/util/setup_common.cpp

@@ -114,7 +114,8 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
     GetMainSignals().RegisterBackgroundSignalScheduler(*g_rpc_node->scheduler);
 
     pblocktree.reset(new CBlockTreeDB(1 << 20, true));
-    g_chainstate = MakeUnique<CChainState>();
+
+    g_chainman.InitializeChainstate();
     ::ChainstateActive().InitCoinsDB(
         /* cache_size_bytes */ 1 << 23, /* in_memory */ true, /* should_wipe */ false);
     assert(!::ChainstateActive().CanFlushToDisk());
@@ -161,7 +162,7 @@ TestingSetup::~TestingSetup()
     m_node.mempool = nullptr;
     m_node.scheduler.reset();
     UnloadBlockIndex();
-    g_chainstate.reset();
+    g_chainman.Reset();
     pblocktree.reset();
 }