[pgpool-hackers: 4332] Refactoring pool_where_to_send

[Tatsuo Ishii]

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.

Best reagards,
--
Tatsuo Ishii
SRA OSS LLC
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