Implement new feature not to accept incoming connections.

Implement new feature not to accept incoming connections.

Currently Pgpool-II accepts up to num_init_children frontends and
queues up more connection requests until one of child process becomes
free. This mostly works well, but if each session take long time, the
queue grows long and the whole system does not work smoothly.

To overcome the problem, I propose a new way to deal with many
connection requests from frontend: just complain and refuses to
connect to Pgpool-II if number of num_init_children is already filled
up. This is exactly the same behavior as PostgreSQL.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2018-November/003153.html

Patch by Tatsuo Ishii.