message: Fix data race in RealmCoordinator
diff: @@ -654,6 +654,7 @@ RealmCoordinator::~RealmCoordinator()
  void RealmCoordinator::unregister_realm(Realm* realm)
  {
  + util::CheckedLockGuard lock(m_realm_mutex);
  // Normally results notifiers are cleaned up by the background worker thread
  // but if that's disabled we need to ensure that any notifiers from this
  // Realm get cleaned up
  @@ -662,7 +663,6 @@ void RealmCoordinator::unregister_realm(Realm* realm)
  clean_up_dead_notifiers();
  }
  {
  - util::CheckedLockGuard lock(m_realm_mutex);
  auto new_end = remove_if(begin(m_weak_realm_notifiers), end(m_weak_realm_notifiers), [=](auto& notifier) {
  return notifier.expired() || notifier.is_for_realm(realm);
  });
  