- fix r2076: removed cache.session_id column, removed DELETEs from cache in session_gc

- trust DB server's time when "touching" cache and messages tables

SQL/mysql.update.sql

@@ -34,10 +34,7 @@ ALTER TABLE `messages`
 -- Updates from version 0.2-beta (InnoDB only)
 
 ALTER TABLE `cache`
-    ADD CONSTRAINT `session_id_fk_cache` FOREIGN KEY (`session_id`)
-    REFERENCES `session`(`sess_id`)
-    ON DELETE CASCADE
-    ON UPDATE CASCADE;
-
+    DROP `session_id`;
+    
 ALTER TABLE `session`
     ADD INDEX `changed_index` (`changed`);