Enhance lifecheck log.

Enhance lifecheck log.

Previously when wd_lifecheck_method = 'query', life checking prints
SQL without application name if "%a" is specified in
log_line_prefix. This commit add application_name "lifecheck_ping" to
make the log looks better.  Since this changes user visible behavior,
I do not apply this to stable branches.

Discussion: [pgpool-hackers: 4603] life check log is not nice
https://www.pgpool.net/pipermail/pgpool-hackers/2025-June/004604.html