BUG/MINOR: backend: Remove CO_FL_SESS_IDLE if a client remains on the last server

When a connection is picked from the session server list because the proxy or
the session are marked to use the last requested server, if it is idle, we must
marked it as used removing the CO_FL_SESS_IDLE flag and decrementing the session
idle_conns counter.

This patch must be backported as far as 1.9.

(cherry picked from commit e91a526c8fe7bf3b655057d41e8b72a2011b7046)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 23133a9bdfa73415a32f6c1cc9644a9e9f5cc37b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/backend.c b/src/backend.c
index 627a4b2..d0d1177 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -663,6 +663,10 @@
 
 						srv = tmpsrv;
 						s->target = &srv->obj_type;
+						if (conn->flags & CO_FL_SESS_IDLE) {
+							conn->flags &= ~CO_FL_SESS_IDLE;
+							s->sess->idle_conns--;
+						}
 						goto out_ok;
 					}
 				}