Deal with idle_session_timeout.

Deal with idle_session_timeout.

If idle_session_timeout (added in PostgreSQL 14) is enabled and the
timeout fires, followings happen:

- If failover_on_backend_error is on (the default), Pgpool-II will
  trigger failover.

- If only one of PostgreSQL servers enables idle_session_timeout,
  Pgpool-II could hang.

To deal with idle_session_timeout detect_idle_session_timeout_error()
is added to detect the error code for idle_session_timeout. If the
error is detected, Pgpool-II returns the error code to frondend as a
fatal error and disconnects the session.  This is a similar fix
implemented for idle_in_transaction_session_timeout.

https://git.postgresql.org/gitweb/?p=pgpool2.git;a=commit;h=3f5986eee360f12e6a0bb77aa46f95abf5f6bc10

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2022-October/004209.html
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