validation: add chainman ref to CChainState

Add an upwards reference to chainstate instances to the owning
ChainstateManager. This is necessary because there are a number
of `this_chainstate == chainman.ActiveChainstate()` checks that
will happen (as a result of assumeutxo) in functions that otherwise
don't have an easily-accessible reference to the chainstate's
ChainManager.

src/validation.h

@@ -601,9 +601,15 @@ public:
     //! CChainState instances.
     BlockManager& m_blockman;
 
+    //! The chainstate manager that owns this chainstate. The reference is
+    //! necessary so that this instance can check whether it is the active
+    //! chainstate within deeply nested method calls.
+    ChainstateManager& m_chainman;
+
     explicit CChainState(
         CTxMemPool* mempool,
         BlockManager& blockman,
+        ChainstateManager& chainman,
         std::optional<uint256> from_snapshot_blockhash = std::nullopt);
 
     /**