sync: Add RecursiveMutex type alias

src/sync.h

@@ -20,7 +20,7 @@
 ////////////////////////////////////////////////
 
 /*
-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 */