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.h

@@ -79,6 +79,8 @@ public:
     void RegisterBackgroundSignalScheduler(CScheduler& scheduler);
     /** Unregister a CScheduler to give callbacks which should run in the background - these callbacks will now be dropped! */
     void UnregisterBackgroundSignalScheduler();
+    /** Call any remaining callbacks on the calling thread */
+    void FlushBackgroundCallbacks();
 
     void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
     void TransactionAddedToMempool(const CTransactionRef &);