Give ZMQ consistent order with UpdatedBlockTip on scheduler thread

Note that UpdatedBlockTip is also used in net_processing to
announce new blocks to peers. As this may need additional review,
this change is included in its own commit.

src/validationinterface.h

@@ -45,7 +45,11 @@ void CallFunctionInValidationInterfaceQueue(std::function<void ()> func);
 
 class CValidationInterface {
 protected:
-    /** Notifies listeners of updated block chain tip */
+    /**
+     * Notifies listeners of updated block chain tip
+     *
+     * Called on a background thread.
+     */
     virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {}
     /**
      * Notifies listeners of a transaction having been added to mempool.