Merge #19927: validation: Reduce direct g_chainman usage

72a1d5c6f3 validation: Remove review-only comments + assertions (Carl Dong)
3756853b15 docs: Move FindFilesToPrune{,Manual} doxygen comment (Carl Dong)
485899a93c style: Make FindFilesToPrune{,Manual} match style guide (Carl Dong)
3f5b5f3f6d validation: Move FindFilesToPrune{,Manual} to BlockManager (Carl Dong)
f8d4975ab3 validation: Move PruneOneBlockFile to BlockManager (Carl Dong)
74f73c783d validation: Pass in chainman to UnloadBlockIndex (Carl Dong)
4668ded6d6 validation: Move ~CMainCleanup logic to ~BlockManager (Carl Dong)

Pull request description:

  This PR paves the way for de-globalizing `g_chainman` entirely by removing the usage of `g_chainman` in the following functions/methods:
  - `~CMainCleanup`
  - `CChainState::FlushStateToDisk`
  - `UnloadBlockIndex`

  The remaining direct uses of `g_chainman` are as follows:
  1. In initialization codepaths:
  	- `AppTests`
  	- `AppInitMain`
  	- `TestingSetup::TestingSetup`
  2. `::ChainstateActive`
  3. `LookupBlockIndex`
  	- Note: `LookupBlockIndex` is used extensively throughout the codebase and require a much larger set of changes, therefore I've left it out of this initial PR

ACKs for top commit:
  MarcoFalke:
    re-ACK 72a1d5c6f3 👚
  jnewbery:
    utACK 72a1d5c6f3

Tree-SHA512: 944a4fa8405eecf39706ff944375d6824373aaeea849d11473f08181eff26b12f70043a8348a5b08e6e9021b243b481842fbdfbc7c3140ca795fce3688b7f5c3

src/init.cpp

@@ -1563,7 +1563,7 @@ bool AppInitMain(const util::Ref& context, NodeContext& node, interfaces::BlockA
                 chainman.m_total_coinstip_cache = nCoinCacheUsage;
                 chainman.m_total_coinsdb_cache = nCoinDBCache;
 
-                UnloadBlockIndex(node.mempool.get());
+                UnloadBlockIndex(node.mempool.get(), chainman);
 
                 // new CBlockTreeDB tries to delete the existing file, which
                 // fails if it's still open from the previous loop. Close it first: