Add an interface to get the queue depth out of CValidationInterface

src/validationinterface.cpp

@@ -54,6 +54,11 @@ void CMainSignals::FlushBackgroundCallbacks() {
     }
 }
 
+size_t CMainSignals::CallbacksPending() {
+    if (!m_internals) return 0;
+    return m_internals->m_schedulerClient.CallbacksPending();
+}
+
 void CMainSignals::RegisterWithMempoolSignals(CTxMemPool& pool) {
     pool.NotifyEntryRemoved.connect(boost::bind(&CMainSignals::MempoolEntryRemoved, this, _1, _2));
 }