Switch to new yesod clientSessionBacked2 for better performance

hledger-web/Foundation.hs

@@ -80,7 +80,9 @@ instance Yesod App where
     -- default session idle timeout is 120 minutes
     makeSessionBackend _ = do
         key <- getKey ".hledger-web_client_session_key.aes"
-        return . Just $ clientSessionBackend key 120
+        let timeout = fromIntegral (120 * 60 :: Int) -- 120 minutes
+        (getCachedDate, _closeDateCacher) <- clientSessionDateCacher timeout
+        return . Just $ clientSessionBackend2 key getCachedDate
 
     defaultLayout widget = do
         master <- getYesod