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.cpp

@@ -6396,6 +6396,17 @@ std::optional<int> ChainstateManager::GetSnapshotBaseHeight() const
     return base ? std::make_optional(base->nHeight) : std::nullopt;
 }
 
+void ChainstateManager::RecalculateBestHeader()
+{
+    AssertLockHeld(cs_main);
+    m_best_header = ActiveChain().Tip();
+    for (auto& entry : m_blockman.m_block_index) {
+        if (!(entry.second.nStatus & BLOCK_FAILED_MASK) && m_best_header->nChainWork < entry.second.nChainWork) {
+            m_best_header = &entry.second;
+        }
+    }
+}
+
 bool ChainstateManager::ValidatedSnapshotCleanup()
 {
     AssertLockHeld(::cs_main);