pgbench: Fix bug in measurement of disconnection delays.

 pgbench: Fix bug in measurement of disconnection delays.
 
 When -C/--connect option is specified, pgbench establishes and closes
 the connection for each transaction. In this case pgbench needs to
 measure the times taken for all those connections and disconnections,
 to include the average connection time in the benchmark result.
 But previously pgbench could not measure those disconnection delays.
 To fix the bug, this commit makes pgbench measure the disconnection
 delays whenever the connection is closed at the end of transaction,
 if -C/--connect option is specified.
 
 Back-patch to v14. Per discussion, we concluded not to back-patch to v13
 or before because changing that behavior in stable branches would
 surprise users rather than providing benefits.
 
 Author: Yugo Nagata
 Reviewed-by: Fabien COELHO, Tatsuo Ishii, Asif Rehman, Fujii Masao
 Discussion: https://postgr.es/m/20210614151155.a393bc7d8fed183e38c9f52a@sraoss.co.jp