PERF: Drop support for gaps in mega topics.

Based on our current implementation, there isn't a
practical way to determine the gaps of large topics
cheaply. We tried to load the gaps in chunks but felt
that the code becomes too complicated. Note that
megatopics are quite rare in the wild.

lib/topic_view.rb

@@ -104,7 +104,14 @@ class TopicView
 
   def gaps
     return unless @contains_gaps
-    @gaps ||= Gaps.new(filtered_post_ids, unfiltered_posts.order(:sort_order).pluck(:id))
+
+    @gaps ||= begin
+      if is_mega_topic?
+        nil
+      else
+        Gaps.new(filtered_post_ids, unfiltered_posts.order(:sort_order).pluck(:id))
+      end
+    end
   end
 
   def last_post