BUG/MINOR: backend: use cum_sess counters instead of cum_conn

This commit is part of a serie to align counters usage between
frontends/listeners on one side and backends/servers on the other.

"stot" metric refers to the total number of sessions. On backend side,
it is interpreted as a number of streams. Previously, this was accounted
using <cum_sess> be_counters field for servers, but <cum_conn> instead
for backend proxies.

Adjust this by using <cum_sess> for both proxies and servers. As such,
<cum_conn> field can be removed from be_counters.

Note that several diagnostic messages which reports total frontend and
backend connections were adjusted to use <cum_sess>. However, this is an
outdated and misleading information as it does reports streams count on
backend side. These messages should be fixed in a separate commit.

This should be backported to all stable releases.

(cherry picked from commit c02ec9a9db205fda124a40a2a87913a0528b0e5b)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit 06053bcad9735562b9eacd6e736e2d71153c665d)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
diff --git a/src/proxy.c b/src/proxy.c
index eeeb8a9..23170d9 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -2004,11 +2004,11 @@
 	 */
 	if ((p->mode == PR_MODE_TCP || p->mode == PR_MODE_HTTP || p->mode == PR_MODE_SYSLOG) && !(p->cap & PR_CAP_INT))
 		ha_warning("Proxy %s stopped (cumulated conns: FE: %lld, BE: %lld).\n",
-			   p->id, p->fe_counters.cum_conn, p->be_counters.cum_conn);
+			   p->id, p->fe_counters.cum_conn, p->be_counters.cum_sess);
 
 	if ((p->mode == PR_MODE_TCP || p->mode == PR_MODE_HTTP) && !(p->cap & PR_CAP_INT))
 		send_log(p, LOG_WARNING, "Proxy %s stopped (cumulated conns: FE: %lld, BE: %lld).\n",
-			 p->id, p->fe_counters.cum_conn, p->be_counters.cum_conn);
+			 p->id, p->fe_counters.cum_conn, p->be_counters.cum_sess);
 
 	if (p->table && p->table->size && p->table->sync_task)
 		task_wakeup(p->table->sync_task, TASK_WOKEN_MSG);