FIX: do not store key tracking last seen time indefinitely

UserStat has some special logic to keep adding time read if repeat calls
are made in intervals less than 100 seconds. This is called regularly
when we update read timings on a topic.

We only need to cache this key in redis for 100 seconds, however previously
we would keep it forever, 1 key per user. This has potential of bloating
a very large amount of keys for no longer active users in redis.

app/models/user_stat.rb

@@ -101,7 +101,7 @@ class UserStat < ActiveRecord::Base
   end
 
   def self.cache_last_seen(id, val)
-    $redis.set(last_seen_key(id), val)
+    $redis.setex(last_seen_key(id), MAX_TIME_READ_DIFF, val)
   end
 
   protected

spec/models/user_stat_spec.rb

@@ -76,6 +76,15 @@ describe UserStat do
     let(:user) { Fabricate(:user) }
     let(:stat) { user.user_stat }
 
+    it 'always expires redis key' do
+      # this tests implementation which is not 100% ideal
+      # that said, redis key leaks are not good
+      stat.update_time_read!
+      ttl = $redis.ttl(UserStat.last_seen_key(user.id))
+      expect(ttl).to be > 0
+      expect(ttl).to be <= UserStat::MAX_TIME_READ_DIFF
+    end
+
     it 'makes no changes if nothing is cached' do
       $redis.del(UserStat.last_seen_key(user.id))
       stat.update_time_read!