Merge bitcoin/bitcoin#25193: indexes: Read the locator's top block during init, allow interaction with reindex-chainstate

97844d9268b87b5d09b1091bfd0326ed18ce5b91 index: Enable reindex-chainstate with active indexes (Martin Zumsande)
60bec3c82d09ec208bf0028ad32499e251ad205e index: Use first block from locator instead of looking for fork point (Martin Zumsande)

Pull request description:

  This makes two improvements to the index init phase:

  **1) Prevent index corruption in case a reorg happens when the index was switched off**:
  This is done by reading in the top block stored in the locator instead of looking for a fork point already in `BaseIndex::Init()`.
  Before, we'd just go back to the fork point by calling `FindForkInGlobalIndex()`, which would have corrupted the coinstatsindex because its saved muhash needs to be reverted step by step by un-applying all blocks in between, which wasn't done before. This is now being done a bit later in  `ThreadSync()`, which has existing logic to call the custom `Rewind()` method when going back along the chain to the forking point (thanks ryanofsky for pointing this out to me!).

  **2) Allow using the `-reindex-chainstate` option without needing to disabling indexes**:
  With `BaseIndex::Init()` not calling `FindForkInGlobalIndex()` anymore, we can allow `reindex-chainstate` with active indexes. `reindex-chainstate` deletes the chain and rebuilds it later in `ThreadImport`, so there is no chain available during `BaseIndex::Init()`, which would lead to problems (see #24789).
  But now we'll only need the chain a bit later in `BaseIndex::ThreadSync`, which will wait for the reindex-chainstate in `ThreadImport` to finish and will continue syncing after that.

ACKs for top commit:
  ryanofsky:
    Code review ACK 97844d9268b87b5d09b1091bfd0326ed18ce5b91. Just simple rebase since last review

Tree-SHA512: e24973fc22e0b87a49026f4820aecb0a4e415f4d381bade9969dd31cf97afecfea0449dce7fcc797343b792199cc8287276d1f5ffa4433dcb54fb24a808db6fb

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