BUG/MEDIUM: queue: Make process_srv_queue return the number of streams

BUG/MEDIUM: queue: Make process_srv_queue return the number of streams

Make process_srv_queue() return the number of streams unqueued, as
pendconn_grab_from_px() did, as that number is used by
srv_update_status() to generate logs.

This should be backported up to 2.6 with
111ea83ed4e13ac3ab028ed5e95201a1b4aa82b8

(cherry picked from commit 5b8899b6ccc7dab3a54a51dcb8ba1512bd0c886c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 70588a16903002709cf3c84255ad8ded73f8e584)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>