Merge bitcoin/bitcoin#21523: validation: run VerifyDB on all chainstates

844ad0ecca doc: IsSnapshotActive (James O'Beirne)
9b604c0207 validation: prepare VerifyDB for assumeutxo (James O'Beirne)
7901647d72 refactor: rename active_chainstate in VerifyDB (James O'Beirne)

Pull request description:

  This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11) (parent PR: #15606)

  ---

  ~~Pretty cut and dry; parameterizes `CVerifyDB` methods so that we can run the verify procedure on multiple chainstates.~~

  Two minor tweaks to ensure that `VerifyDB` can be run on multiple chainstates and a corresponding rename.

ACKs for top commit:
  fjahr:
    Code review re-ACK 844ad0ecca
  MarcoFalke:
    review ACK 844ad0ecca 🐥

Tree-SHA512: 26a398cf4dabc1aa0850743921dba0452b4813848a3c777586dc981716737e98e17b8110254a5c41af95dd236e0c00dc8b4eee891d69bef825a5e1911fc499d0

src/validation.h

@@ -329,7 +329,12 @@ class CVerifyDB {
 public:
     CVerifyDB();
     ~CVerifyDB();
-    bool VerifyDB(const CChainParams& chainparams, CChainState& active_chainstate, CCoinsView *coinsview, int nCheckLevel, int nCheckDepth) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    bool VerifyDB(
+        CChainState& chainstate,
+        const CChainParams& chainparams,
+        CCoinsView& coinsview,
+        int nCheckLevel,
+        int nCheckDepth) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 };
 
 enum DisconnectResult
@@ -913,6 +918,8 @@ public:
         return m_blockman.m_block_index;
     }
 
+    //! @returns true if a snapshot-based chainstate is in use. Also implies
+    //!          that a background validation chainstate is also in use.
     bool IsSnapshotActive() const;
 
     std::optional<uint256> SnapshotBlockhash() const;