message: Correctly add free list to modified blocks
diff: @@ -247,6 +247,7 @@ void SingleFileBlockManager::WriteHeader(DatabaseHeader header) {
  free_list.erase(entry);
  }
  header.free_list = writer.block->id;
  + modified_blocks.push_back(writer.block->id);
  writer.Write<uint64_t>(free_list.size());
  for (auto &block_id : free_list) {
  