Combine pg_walinspect till_end_of_wal functions with others

Hi,

In a recent discussion [1], Michael Paquier asked if we can combine
pg_walinspect till_end_of_wal functions with other functions
pg_get_wal_records_info and pg_get_wal_stats. The code currently looks
much duplicated and the number of functions that pg_walinspect exposes
to the users is bloated. The point was that the till_end_of_wal
functions determine the end LSN and everything else that they do is
the same as their counterpart functions. Well, the idea then was to
keep things simple, not clutter the APIs, have better and consistent
user-inputted end_lsn validations at the cost of usability and code
redundancy. However, now I tend to agree with the feedback received.

I'm attaching a patch doing the $subject with the following behavior:
1. If start_lsn is NULL, error out/return NULL.
2. If end_lsn isn't specified, default to NULL, then determine the end_lsn.
3. If end_lsn is specified as NULL, then determine the end_lsn.
4. If end_lsn is specified as non-NULL, then determine if it is
greater than start_lsn if yes, go ahead do the job, otherwise error
out.

Another idea is to convert till_end_of_wal flavors to SQL-only
functions and remove the c code from pg_walinspect.c. However, I
prefer $subject and completely remove till_end_of_wal flavors for
better usability in the long term.

Thoughts?

[1] https://www.postgresql.org/message-id/CALj2ACV-WBN%3DEUgUPyYOGitp%2Brn163vMnQd%3DHcWrnKt-uqFYFA%40mail.gmail.com

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