Fix around conn_duration in pgbench

Hi,

TState has a field called "conn_duration" and this is, the comment says,
"cumulated connection and deconnection delays". This value is summed over
threads and reported as "average connection time" under -C/--connect.
If this options is not specified, the value is never used.

However, I found that conn_duration is calculated even when -C/--connect
is not specified, which is waste. SO we can remove this code as fixed in
the attached patch.

In addition, deconnection delays are not cumulated even under -C/--connect
in spite of mentioned in the comment. I also fixed this in the attached patch.

Regards,
Yugo Nagata

--
Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>