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

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outdated and misleading information as it does reports streams count on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.