MINOR: stats: Enhancement to stats page to provide information of last session time.

Summary:
Track and report last session time on the stats page for each server
in every backend, as well as the backend.

This attempts to address the requirement in the ROADMAP

  - add a last activity date for each server (req/resp) that will be
    displayed in the stats. It will be useful with soft stop.

The stats page reports this as time elapsed since last session. This
change does not adequately address the requirement for long running
session (websocket, RDP... etc).
diff --git a/include/types/counters.h b/include/types/counters.h
index efb48f6..ecdc7cb 100644
--- a/include/types/counters.h
+++ b/include/types/counters.h
@@ -29,6 +29,7 @@
 	long long    cum_conn;                  /* cumulated number of received connections */
 	long long    cum_sess;                  /* cumulated number of accepted connections */
 	long long  cum_lbconn;                  /* cumulated number of sessions processed by load balancing (BE only) */
+	unsigned long last_sess;                /* last session time */
 
 	unsigned int cps_max;                   /* maximum of new connections received per second */
 	unsigned int sps_max;                   /* maximum of new connections accepted per second (sessions) */
@@ -85,6 +86,7 @@
 
 	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 */
+	unsigned long last_sess;                 /* last session time */
 
 	long long bytes_in;			/* number of bytes transferred from the client to the server */
 	long long bytes_out;			/* number of bytes transferred from the server to the client */