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.

(cherry picked from commit dc9ce9c26469e00ab71fe6387dbd13010d4930f0)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 1385e4ca16b3797b0091a959b626935cd7f29b38)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>