message: refactor: add thread safety lock assertion to WriteBlockIndexDB()
  The new helper function, BlockManager::WriteBlockIndexDB(),
  has a thread safety lock annotation in its declaration but is
  missing the corresponding run-time lock assertion in its definition.
  Per doc/developer-notes.md: "Combine annotations in function
  declarations with run-time asserts in function definitions."
diff: @@ -348,6 +348,7 @@ void BlockManager::Unload()
  bool BlockManager::WriteBlockIndexDB()
  {
  + AssertLockHeld(::cs_main);
  std::vector<std::pair<int, const CBlockFileInfo*>> vFiles;
  vFiles.reserve(m_dirty_fileinfo.size());
  for (std::set<int>::iterator it = m_dirty_fileinfo.begin(); it != m_dirty_fileinfo.end();) {
  