[MINOR] compute the max of sessions/s on fe/be/srv

Some users want to keep the max sessions/s seen on servers, frontends
and backends for capacity planning. It's easy to grab it while the
session count is updated, so let's keep it.
diff --git a/include/proto/server.h b/include/proto/server.h
index e05a4ac..7479c2e 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -40,6 +40,8 @@
 {
 	s->cum_sess++;
 	update_freq_ctr(&s->sess_per_sec, 1);
+	if (s->sess_per_sec.curr_ctr > s->sps_max)
+		s->sps_max = s->sess_per_sec.curr_ctr;
 }
 
 #endif /* _PROTO_SERVER_H */