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/init.cpp

@@ -1525,11 +1525,8 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
                             break;
                         }
 
-                        // Only verify the DB of the active chainstate. This is fixed in later
-                        // work when we allow VerifyDB to be parameterized by chainstate.
-                        if (&::ChainstateActive() == chainstate &&
-                            !CVerifyDB().VerifyDB(
-                                chainparams, *chainstate, &chainstate->CoinsDB(),
+                        if (!CVerifyDB().VerifyDB(
+                                *chainstate, chainparams, chainstate->CoinsDB(),
                                 args.GetArg("-checklevel", DEFAULT_CHECKLEVEL),
                                 args.GetArg("-checkblocks", DEFAULT_CHECKBLOCKS))) {
                             strLoadError = _("Corrupted block database detected");