MEDIUM: fwlc: Make it so fwlc_srv_reposition works with unqueued srv

MEDIUM: fwlc: Make it so fwlc_srv_reposition works with unqueued srv

Modify fwlc_srv_reposition() so that it does not assume that the server
was already queued, and so make it so it works even if s->tree_elt is
NULL.
While the server will usually be queued, there is an unlikely
possibility that when the server attempted to get queued when it got up,
it failed due to a memory allocation failure, and it just expect the
server_requeue tasklet to run to take care of that later.

This should be backported to 3.2.
This is part of an attempt to fix github issue #3143