Move blockstorage-related unload to BlockManager::Unload

This is a refactor and safe to do because:
* UnloadBlockIndex calls ChainstateManager::Unload, which calls
  BlockManager::Unload
* Only unit tests call Unload directly

src/node/blockstorage.cpp

@@ -348,6 +348,11 @@ void BlockManager::Unload()
     }
 
     m_block_index.clear();
+
+    vinfoBlockFile.clear();
+    nLastBlockFile = 0;
+    setDirtyBlockIndex.clear();
+    setDirtyFileInfo.clear();
 }
 
 bool BlockManager::WriteBlockIndexDB()