validation: add RecalculateBestHeader() function

It recalculates m_best_header by looping over the entire
block index. Even though this is not very performant, it
will only be used in rare situations that cannot be
triggered by others without a cost:
As part of to invalidateblock / reconsiderblock rpcs, or when a
block with an accepted header with valid PoW turns out to be invalid
later during full validation.

src/validation.h

@@ -1321,6 +1321,11 @@ public:
     //! nullopt.
     std::optional<int> GetSnapshotBaseHeight() const EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    //! If, due to invalidation / reconsideration of blocks, the previous
+    //! best header is no longer valid / guaranteed to be the most-work
+    //! header in our block-index not known to be invalid, recalculate it.
+    void RecalculateBestHeader() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     CCheckQueue<CScriptCheck>& GetCheckQueue() { return m_script_check_queue; }
 
     ~ChainstateManager();