Fix blockTrackerInspectorMiddleware (#88)

The final line of this middleware should be `return undefined;`, not `return await next();`, as `next()` is already called earlier in the function.

src/block-tracker-inspector.ts

@@ -38,6 +38,6 @@ export function createBlockTrackerInspectorMiddleware({
         await blockTracker.checkForLatestBlock();
       }
     }
-    return next();
+    return undefined;
   });
 }