message: sync: Add RecursiveMutex type alias
diff: ////////////////////////////////////////////////
  /*
  -CCriticalSection mutex;
  +RecursiveMutex mutex;
  std::recursive_mutex mutex;
  LOCK(mutex);
  @@ -104,6 +104,7 @@ public:
  * Wrapped mutex: supports recursive locking, but no waiting
  * TODO: We should move away from using the recursive lock by default.
  */
  +using RecursiveMutex = AnnotatedMixin<std::recursive_mutex>;
  typedef AnnotatedMixin<std::recursive_mutex> CCriticalSection;
  /** Wrapped mutex: supports waiting but not recursive locking */
  