PostgreSQL: add fields for pooling

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: add connection pooling support

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: reclaim connections when it is pooled

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: add pool methods to postgresql object

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: fix minor bugs

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: init config list

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: copy the terminate NULL

Signed-off-by: swpd <swpdtz@gmail.com>
PostgreSQL: fix connection remove mistake

Signed-off-by: swpd <swpdtz@gmail.com>