DEV: Topic tracking state improvements (#12958)

Original PR which had to be reverted **https://github.com/discourse/discourse/pull/12555**. See the description there for what this PR is achieving, plus below.

The issue with the original PR is addressed in 92ef54f402

If you went to the `x unread` link for a tag Chrome would freeze up and possibly crash, or eventually unfreeze after nearly 10 mins. Other routes for unread/new were similarly slow. From profiling the issue was the `sync` function of `topic-tracking-state.js`, which calls down to `isNew` which in turn calls `moment`, a change I had made in the PR above. The time it takes locally with ~1400 topics in the tracking state is 2.3 seconds.

To solve this issue, I have moved these calculations for "created in new period" and "unread not too old" into the tracking state serializer.

When I was looking at the profiler I also noticed this issue which was just compounding the problem. Every time we modify topic tracking state we recalculate the sidebar tracking/everything/tag counts. However this calls `forEachTracked` and `countTags` which can be quite expensive as they go through the whole tracking state (and were also calling the removed moment functions).

I added some logs and this was being called 30 times when navigating to a new /unread route because  `sync` is being called from `build-topic-route` (one for each topic loaded due to pagination). So I just added a debounce here and it makes things even faster.

Finally, I changed topic tracking state to use a Map so our counts of the state keys is faster (Maps have .size whereas objects you have to do Object.keys(obj) which is O(n).)

app/serializers/current_user_serializer.rb

@@ -44,6 +44,7 @@ class CurrentUserSerializer < BasicUserSerializer
              :read_faq,
              :automatically_unpin_topics,
              :mailing_list_mode,
+             :treat_as_new_topic_start_date,
              :previous_visit_at,
              :seen_notification_id,
              :primary_group_id,
@@ -278,6 +279,10 @@ class CurrentUserSerializer < BasicUserSerializer
     object.user_option.mailing_list_mode
   end
 
+  def treat_as_new_topic_start_date
+    object.user_option.treat_as_new_topic_start_date
+  end
+
   def skip_new_user_tips
     object.user_option.skip_new_user_tips
   end