BUG/MEDIUM: queues: Make sure we call process_srv_queue() when leaving

BUG/MEDIUM: queues: Make sure we call process_srv_queue() when leaving

In stream_free(), make sure we call process_srv_queue() each time we
call sess_change_server(), otherwise a server may end up not dequeuing
any stream when it could do so. In some extreme cases it could lead to
an infinite loop, as the server would appear to be available, as its
"served" parameter would be < maxconn, but would end up not being used,
as there are elements still in its queue.

This should be backported up to 2.6.