[New-Feature] Dynamic spare process management

[New-Feature] Dynamic spare process management

This feature allows selecting between static and dynamic process management modes.
Static process management is the same as the existing behavior of Pgpool-II, where it
spawns all child processes at startup. The new Dynamic mode keeps track of idle
processes and forks or kills processes to keep this number within
the specified boundaries.

Four new settings, process_management_mode,  process_management_strategy,
min_spare_children, and max_spare_children are added to configure the process
management behavior, while  process_management_strategy allows selecting
between three possible scaling-down strategies.

The first version of the patch was shared by "zhoujianshen at highgo.com" and reworked by me
Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2022-September/004189.html
Reviewed by: Bo Peng and Tatsuo Ishii