Fix for failover.

I got a problem using roundcube in a failover server (2 sctrutures).

The problem is that, there are some differences in data between this two structures(such as user ids), if there is a user logged, and one of structures fails.The current user could be changed for another user, because the user_id stored in SESSION.

![roundcube 1 ](https://f.cloud.github.com/assets/308606/90918/a48d2ffa-6587-11e2-9b8a-08c8c35b5cfd.png)