MINOR: lbprm: add a new callback ->server_requeue to the lbprm

MINOR: lbprm: add a new callback ->server_requeue to the lbprm

This callback will be used to reposition a server to its expected
position regardless of the fact that it was taken or dropped. It
will only be used by supporting LB algos. For now, only fwlc defines
it and assigns it to fwlc_srv_reposition(). At the moment it's not
used yet.