Re: allow spread checkpoints when changing checksums online

Hi,

Here's a v6 addressing the comments. I've split it into two parts:

0001 - While updating the "Impact on System of Online Operations" part,
I realized it does not mention checkpoints at all. Which seems wrong,
because even on 19 it can be a source of disruption. So 0001 adds this
reference, and I intend to backpatch this to 19.

FWIW the "Impact on System of Online Operations" seems a bit strange to
me. It's as if it was about impact on "System of Online Operations",
which does not seem right. I think it should be "Impact of Online
Operations on System" instead.

The other thing I noticed is that wal.sgml does not quite follow the
terminology, it talks about "checksums" and not "data checksums" in a
bunch of places. Maybe it should be cleaned up, but I haven't done
anything about that.

0002 - This is the main part for 20. It now acquires the locks even in
cases where I think it's not strictly necessary (but explaining the
reasoning would be longer than just following the locking protocol). It
also uses the launch_fast_checkpoint in launcher_exit, as explained in
the previous message. It should also fix all the typos, etc.

regards

--
Tomas Vondra