Add new field "load_balance_node" to "SHOW pool_pools" and pcp_proc_info.

Add new field "load_balance_node" to "SHOW pool_pools" and pcp_proc_info.

The new field is "1" if pgpool process is connected by a client and
the session uses the backend id as a load balance node. Users can
execute the commands to find out if there's any session that uses the
backend as the load balance node. If so, shutting down the backend may
cause session disconnection.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2023-July/004353.html

Note: I accidentally pushed only doc part of the commit in 4658f84870e8edfd39920f273bab1a12d71d8986.
This is a follow-up commit for actual codes.