PERF: reduce queries required for post timings

- also freezes a bunch of strings
- bypass active record for an exists query

spec/models/user_stat_spec.rb

@@ -77,22 +77,26 @@ describe UserStat do
     let(:stat) { user.user_stat }
 
     it 'makes no changes if nothing is cached' do
-      stat.expects(:last_seen_cached).returns(nil)
+      $redis.del(UserStat.last_seen_key(user.id))
       stat.update_time_read!
       stat.reload
       expect(stat.time_read).to eq(0)
     end
 
     it 'makes a change if time read is below threshold' do
-      stat.expects(:last_seen_cached).returns(Time.now - 10)
+      freeze_time
+      UserStat.cache_last_seen(user.id, (Time.now - 10).to_f)
       stat.update_time_read!
       stat.reload
       expect(stat.time_read).to eq(10)
     end
 
     it 'makes no change if time read is above threshold' do
+      freeze_time
+
       t = Time.now - 1 - UserStat::MAX_TIME_READ_DIFF
-      stat.expects(:last_seen_cached).returns(t)
+      UserStat.cache_last_seen(user.id, t.to_f)
+
       stat.update_time_read!
       stat.reload
       expect(stat.time_read).to eq(0)