allow spread checkpoints when changing checksums online

Hi,

Here's a small patch re-introducing the option to use spread checkpoints
(instead of always using CHECKPOINT_FAST) for online checksum changes.

The version v20251201 posted in [1]/messages/by-id/477897AE-1314-4724-9694-0BABC4F4ABDA@yesql.se supported this, but the next patch
version was without checkpoints and so removed the "fast" parameter too.
Then we realized the checkpoints are actually needed, but were added
back to keep it as simple as possible. Or maybe it was an omission, not
sure, and there's no explanation on the thread.

I recall someone claiming always doing fast checkpoints is fine, because
we've already written the whole database into WAL anyway, and on large
databases that's likely way more expensive than a single checkpoint. I
don't buy that, for two reasons:

- We do have throttling for the rewrite phase, thanks to the cost_limit
and cost_delay parameters. So we can effectively throttle it, to reduce
impact of the checksums change. In which case the "fast" checkpoint can
be way more disruptive.

- We need to do checkpoints even when "disabling" checksums, in which
case we don't rewrite any data pages (or WAL-log anything), we just need
to persist the new checksum state. Which just makes the fast checkpoint
relatively more disruptive.

The attached patch is mostly extracted from v20251201, and adds the
"fast" parameter back to pg_{enable,disable}_data_checksums.

I have two open questions regarding it:

1) What should be the default? I've used fast=true, mostly because
that's what PG19 is going to do (fast checkpoints by default). It's also
somewhat consistent with e.g. VACUUM which does no throttling by
default. But I assume most production uses would want fast=false?

2) I haven't adjusted the TAP tests. We could use fast=false in a couple
of the test_checksums tests, but I'm not sure it's worth it and it makes
it way more time consuming.

We could reduce checkpoint_timeout to something very aggressive. And in
fact that's what I did locally with the TAP tests I posted in [2]/messages/by-id/9e1331e1-93a0-4e27-934a-17b89342be4d@vondra.me. But
I'm still not convinced it's worth it - the checkpoints are still
synchronous, of course.

regards

[1]: /messages/by-id/477897AE-1314-4724-9694-0BABC4F4ABDA@yesql.se
/messages/by-id/477897AE-1314-4724-9694-0BABC4F4ABDA@yesql.se

[2]: /messages/by-id/9e1331e1-93a0-4e27-934a-17b89342be4d@vondra.me
/messages/by-id/9e1331e1-93a0-4e27-934a-17b89342be4d@vondra.me

--
Tomas Vondra