Do not unlock cs_main in ABC unless we've actually made progress.

Technically, some internal datastructures may be in an inconsistent
state if we do this, though there are no known bugs there. Still,
for future safety, its much better to only unlock cs_main if we've
made progress (not just tried a reorg which may make progress).

src/validationinterface.h

@@ -61,7 +61,11 @@ protected:
      */
     ~CValidationInterface() = default;
     /**
-     * Notifies listeners of updated block chain tip
+     * Notifies listeners when the block chain tip advances.
+     *
+     * When multiple blocks are connected at once, UpdatedBlockTip will be called on the final tip
+     * but may not be called on every intermediate tip. If the latter behavior is desired,
+     * subscribe to BlockConnected() instead.
      *
      * Called on a background thread.
      */