validation: Load pindexBestHeader in ChainMan

Now BlockManager::LoadBlockIndex() will ACTUALLY only load BlockMan
members.

[META] In a later commit, pindexBestHeader will be moved to ChainMan as
       a member

-----

Code Reviewer Notes

Call graph of relevant functions:

ChainstateManager::LoadBlockIndex() <-- Moved to
    calls BlockManager::LoadBlockIndexDB()
        which calls BlockManager::LoadBlockIndex() <-- Moved from

There is only one call to each of inner functions, meaning that no
behavior is changing.

src/validation.cpp

@@ -4203,6 +4203,8 @@ bool ChainstateManager::LoadBlockIndex()
             if (pindex->nStatus & BLOCK_FAILED_MASK && (!m_best_invalid || pindex->nChainWork > m_best_invalid->nChainWork)) {
                 m_best_invalid = pindex;
             }
+            if (pindex->IsValid(BLOCK_VALID_TREE) && (pindexBestHeader == nullptr || CBlockIndexWorkComparator()(pindexBestHeader, pindex)))
+                pindexBestHeader = pindex;
         }
 
         needs_init = m_blockman.m_block_index.empty();