validation: add CChainState.m_from_snapshot_blockhash

This parameter is unused, but in future commits will allow ChainstateManager to
differentiate between chainstates created from a UTXO snapshot from those that
weren't.

src/validation.h

@@ -591,8 +591,8 @@ private:
     std::unique_ptr<CoinsViews> m_coins_views;
 
 public:
-    CChainState(BlockManager& blockman) : m_blockman(blockman) {}
-    CChainState();
+    explicit CChainState(BlockManager& blockman) : m_blockman(blockman) {}
+    explicit CChainState(uint256 from_snapshot_blockhash = uint256());
 
     /**
      * Initialize the CoinsViews UTXO set database management data structures. The in-memory
@@ -620,6 +620,13 @@ public:
     //! @see CChain, CBlockIndex.
     CChain m_chain;
 
+    /**
+     * The blockhash which is the base of the snapshot this chainstate was created from.
+     *
+     * IsNull() if this chainstate was not created from a snapshot.
+     */
+    const uint256 m_from_snapshot_blockhash{};
+
     /**
      * The set of all CBlockIndex entries with BLOCK_VALID_TRANSACTIONS (for itself and all ancestors) and
      * as good as our current tip or better. Entries may be failed, though, and pruning nodes may be