Introduce a new view for checkpointer related stats

Hi,

pg_stat_bgwriter view currently reports checkpointer stats as well. It
is that way because historically checkpointer was part of bgwriter
until the commits 806a2ae and bf405ba, that went into PG 9.2,
separated them out. I think it is time for us to separate checkpointer
stats to its own view. I discussed it in another thread [1] and it
seems like there's an unequivocal agreement for the proposal.

I'm attaching a patch introducing a new pg_stat_checkpointer view,
with this change the pg_stat_bgwriter view only focuses on bgwriter
related stats. The patch does mostly mechanical changes. I'll add it
to CF in a bit.

Thoughts?

[1]

https://www.postgresql.org/message-id/flat/20221116181433.y2hq2pirtbqmmndt%40awork3.anarazel.de#b873a4bd7d8d7ec70750a7047db33f56
https://www.postgresql.org/message-id/CA%2BTgmoYCu6RpuJ3cZz0e7cZSfaVb%3Dcr6iVcgGMGd5dxX0MYNRA%40mail.gmail.com

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