[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/types/server.h b/include/types/server.h
index a52a9d8..7c7a54e 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -124,6 +124,7 @@
 	long long retries, redispatches;		/* retried and redispatched connections */
 	long long failed_secu;			/* blocked responses because of security concerns */
 	struct freq_ctr sess_per_sec;		/* sessions per second on this server */
+	unsigned int sps_max;			/* maximum of new sessions per second seen on this server */
 	long long cum_sess;			/* cumulated number of sessions really sent to this server */
 	long long cum_lbconn;			/* cumulated number of sessions directed by load balancing */