Make loadbalancer direct user to previously used server

[Arthur Lutz]

It would be a nice feature to have the loadbalancer direct a user a to a previously used server. This reduces the utility of load balancing but in certain use cases can be useful. We keep the fail-over feature that if the "previously used server" is down, the user can still log on to another server (transparently).

We have the case of users using local /opt disc on servers for IO intensive tasks and so we want them to end up on the same server every time (unless it's down).