Helper functions for wait_for_catchup() in Cluster.pm

Hi hackers,

please find attached a patch proposal to define $SUBJECT.

The idea has been raised in [1], where we are adding more calls to wait_for_catchup() in 'replay' mode.

The current code already has 25 of those, so the proposed patch is defining a new wait_for_replay_catchup() function.

While at it, adding also:

- wait_for_write_catchup(): called 5 times
- wait_for_sent_catchup() and wait_for_flush_catchup() for consistency purpose (while there is
currently no occurrences of wait_for_catchup() in 'sent' or 'flush' mode.).

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

[1]: https://www.postgresql.org/message-id/20230106034036.2m4qnn7ep7b5ipet%40awork3.anarazel.de