PERF: Simplify query of `UserStat#update_topic_reply_count`.

PERF: Simplify query of UserStat#update_topic_reply_count.

For a user with alot of posts, we get a 25% speed up.

diff --git a/app/models/user_stat.rb b/app/models/user_stat.rb
index e512a94..a1b39d5 100644
--- a/app/models/user_stat.rb
+++ b/app/models/user_stat.rb
@@ -129,15 +129,11 @@ class UserStat < ActiveRecord::Base
   # topic_reply_count is a count of posts in other users' topics
   def update_topic_reply_count
     self.topic_reply_count =
-        Topic
-      .where(['id in (
-              SELECT topic_id FROM posts p
-              JOIN topics t2 ON t2.id = p.topic_id
-              WHERE p.deleted_at IS NULL AND
-                t2.user_id <> p.user_id AND
-                p.user_id = ?
-              )', self.user_id])
-      .count
+      Topic
+        .joins("INNER JOIN posts ON topics.id = posts.topic_id AND topics.user_id <> posts.user_id")
+        .where("posts.deleted_at IS NULL AND posts.user_id = ?", self.user_id)
+        .distinct
+        .count
   end
 
   MAX_TIME_READ_DIFF = 100

GitHub sha: f0f3deb3