Refactor pool_where_to_send.

Refactor pool_where_to_send.

pool_where_to_send() is a module in charge of judging where to a query
in terms of load balancing.  It had been already big and I felt
difficulty to maintain it. Therefore I extracted two relatively large
code path, namely for streaming replication mode and native
replication mode (plus snapshot isolation mode) into
where_to_send_main_replica() and where_to_send_native_replication()
respectively.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2023-May/004333.html