Various optimizations:

o Add model_cache::get() which caches models avoiding duplicate lookups
o Stop using ORM relationships for Item_Model::owner so that we can use caching
o For Item_Model::xxx_edit fields, don't make them editable for guests
o Other minor stuff.

These optimizations reduce the number of queries for a 9-photos page from ~200
to ~45.  Still way too many!

modules/user/helpers/user.php

@@ -73,7 +73,7 @@ class user_Core {
    * @return User_Model
    */
   static function guest() {
-    return ORM::factory("user", 1);
+    return model_cache::get("user", 1);
   }
 
   /**