MINOR: stats: Report max times in addition of the averages for sessions

Now, for the sessions, the maximum times (queue, connect, response, total) are
reported in addition of the averages over the last 1024 connections. These
values are called qtime_max, ctime_max, rtime_max and ttime_max.

This patch is related to #272.
diff --git a/include/types/stats.h b/include/types/stats.h
index 52ea18d..4d2a933 100644
--- a/include/types/stats.h
+++ b/include/types/stats.h
@@ -430,6 +430,10 @@
 	ST_F_CACHE_HITS,
 	ST_F_SRV_ICUR,
 	ST_F_SRV_ILIM,
+	ST_F_QT_MAX,
+	ST_F_CT_MAX,
+	ST_F_RT_MAX,
+	ST_F_TT_MAX,
 
 	/* must always be the last one */
 	ST_F_TOTAL_FIELDS