pgbench: Avoid unnecessary measurement of connection delays.

 pgbench: Avoid unnecessary measurement of connection delays.
 
 Commit 547f04e734 changed pgbench so that it used the measurement result
 of connection delays in its benchmark report only when -C/--connect option
 is specified. But previously those delays were unnecessarily measured
 even when that option is not specified. Which was a waste of cycles.
 This commit improves pgbench so that it avoids such unnecessary measurement.
 
 Back-patch to v14 where commit 547f04e734 first appeared.
 
 Author: Yugo Nagata
 Reviewed-by: Fabien COELHO, Asif Rehman, Fujii Masao
 Discussion: https://postgr.es/m/20210614151155.a393bc7d8fed183e38c9f52a@sraoss.co.jp