Add an interface to get the queue depth out of CValidationInterface

src/scheduler.h

@@ -108,6 +108,8 @@ public:
     // Processes all remaining queue members on the calling thread, blocking until queue is empty
     // Must be called after the CScheduler has no remaining processing threads!
     void EmptyQueue();
+
+    size_t CallbacksPending();
 };
 
 #endif