message: Adjust locking a little to avoid warnings with older versions of clang
diff: @@ -963,10 +963,9 @@ void RealmCoordinator::run_async_notifiers()
  for (auto& notifier : notifiers)
  notifier->run();
  - lock.lock();
  + util::CheckedLockGuard lock(m_notifier_mutex);
  for (auto& notifier : notifiers)
  notifier->prepare_handover();
  - lock.unlock();
  }
  // Advance the non-new notifiers to the same version as we advanced the new
  @@ -991,7 +990,7 @@ void RealmCoordinator::run_async_notifiers()
  // Reacquire the lock while updating the fields that are actually read on
  // other threads
  - lock.lock();
  + util::CheckedLockGuard lock2(m_notifier_mutex);
  for (auto& notifier : new_notifiers) {
  notifier->prepare_handover();
  }
  