PERF: Reduce records queried in UserStat.update_first_unread_pm. (#15016)

The inefficiency here is that we were previously fetching all the
records from `TopicAllowedUser` before filtering against a limited subset of
users based on `User#last_seen_at`.

app/models/user_stat.rb

@@ -22,10 +22,6 @@ class UserStat < ActiveRecord::Base
     DB.exec(<<~SQL, archetype: Archetype.private_message, now: UPDATE_UNREAD_MINUTES_AGO.minutes.ago, last_seen: last_seen, limit: limit)
     UPDATE user_stats us
     SET first_unread_pm_at = COALESCE(Z.min_date, :now)
-    FROM (
-      SELECT
-        Y.user_id,
-        Y.min_date
     FROM (
       SELECT
         u1.id user_id,
@@ -47,10 +43,7 @@ class UserStat < ActiveRecord::Base
                                        ELSE t.highest_post_number
                                        END
         AND (COALESCE(tu.notification_level, 1) >= 2)
-          GROUP BY tau.user_id
-        ) AS X ON X.user_id = u1.id
-      ) AS Y
-      WHERE Y.user_id IN (
+        AND tau.user_id IN (
           SELECT id
           FROM users
           WHERE last_seen_at IS NOT NULL
@@ -58,6 +51,8 @@ class UserStat < ActiveRecord::Base
           ORDER BY last_seen_at DESC
           LIMIT :limit
         )
+        GROUP BY tau.user_id
+      ) AS X ON X.user_id = u1.id
     ) AS Z
     WHERE us.user_id = Z.user_id
     SQL