Flush CValidationInterface callbacks prior to destruction

Note that the CScheduler thread cant be running at this point,
it has already been stopped with the rest of the init threadgroup.
Thus, just calling any remaining loose callbacks during Shutdown()
is sane.

src/validationinterface.cpp

@@ -44,6 +44,10 @@ void CMainSignals::UnregisterBackgroundSignalScheduler() {
     m_internals.reset(nullptr);
 }
 
+void CMainSignals::FlushBackgroundCallbacks() {
+    m_internals->m_schedulerClient.EmptyQueue();
+}
+
 CMainSignals& GetMainSignals()
 {
     return g_signals;