Add health check statistics collection/display feature.

Add health check statistics collection/display feature.

The health check process now collects statistics data such as number
of total health check performed, number of health check retry count
and health check duration and so on in the shared memory area. This
commit also adds new "show pool_health_check_stats" SQL
command. Corresponding PCP command and pgpool_adm function will be
added in subsequent commit.