stream_topic_history: Let server help us with latest msg id for topics.

If we already have latest data for topics, we shouldn't pass on the
opportunity to update the latest message id for existing topics.

web/src/stream_topic_history.ts

@@ -187,8 +187,10 @@ export class PerStreamHistory {
             const existing = this.topics.get(topic_name);
 
             if (existing) {
-                // Trust out local data more, since it
-                // maintains counts.
+                // If we have a topic in our cache, we update
+                // the message_id to accurately reflect the latest
+                // message in the topic.
+                existing.message_id = message_id;
                 continue;
             }