Log checkpoint request flags in checkpoint completion messages.

Log checkpoint request flags in checkpoint completion messages.

Checkpoint completion log messages include more detail than checkpoint
start messages, but previously omitted the checkpoint request flags,
which were only logged at checkpoint start. As a result, users had to
correlate completion messages with earlier start messages to see
the full context.

This commit includes the checkpoint request flags in the checkpoint
completion log message as well. This duplicates some information,
but makes the completion message self-contained and easier to interpret.

Author: Soumya S Murali <soumyamurali.work@gmail.com>
Reviewed-by: Michael Banck <mbanck@gmx.net>
Reviewed-by: Yuan Li <carol.li2025@outlook.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAMtXxw9tPwV=NBv5S9GZXMSKPeKv5f9hRhSjZ8__oLsoS5jcuA@mail.gmail.com