MEDIUM: session: maintain per-backend and per-server time statistics

Using the last rate counters, we now compute the queue, connect, response
and total times per server and per backend with a 95% accuracy over the last
1024 samples. The operation is cheap so we don't need to condition it.
diff --git a/src/proto_http.c b/src/proto_http.c
index 082f161..48dbc43 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -4714,6 +4714,8 @@
 		s->do_log(s);
 	}
 
+	session_update_time_stats(s);
+
 	s->logs.accept_date = date; /* user-visible date for logging */
 	s->logs.tv_accept = now;  /* corrected date for internal use */
 	tv_zero(&s->logs.tv_request);