Add an interface to get the queue depth out of CValidationInterface

src/scheduler.cpp

@@ -206,3 +206,8 @@ void SingleThreadedSchedulerClient::EmptyQueue() {
         should_continue = !m_callbacks_pending.empty();
     }
 }
+
+size_t SingleThreadedSchedulerClient::CallbacksPending() {
+    LOCK(m_cs_callbacks_pending);
+    return m_callbacks_pending.size();
+}