Move pindexBestInvalid to ChainstateManager

A private member is better than a global.

src/validation.h

@@ -866,6 +866,9 @@ private:
     //! by the background validation chainstate.
     bool m_snapshot_validated{false};
 
+    CBlockIndex* m_best_invalid;
+    friend bool BlockManager::LoadBlockIndex(const Consensus::Params&, ChainstateManager&);
+
     //! Internal helper for ActivateSnapshot().
     [[nodiscard]] bool PopulateAndValidateSnapshot(
         CChainState& snapshot_chainstate,