Re: allow spread checkpoints when changing checksums online

Hi,

Here's a rebased patch, addressing the minor bitrot, and with some very
minor adjustments.

1) proper commit message

2) cleanup / rewording in the sgml docs

3) I've noticed the SGML docs don't have default values for the existing
parameters (cost_delay, cost_limit), so I added those. Maybe backpatch?

4) I've added the new 'fast' parameter to the DataChecksums module, but
mostly just for completeness. The TAP tests still use the default value,
so that it doesn't take longer.

I've spent some time thinking about the default value. For a while I
thought maybe it should default to 'false' (in which case we use spread
checkpoints, which is the less disruptive option).

But that's actually futile, because the cost_delay still defaults to 0,
which means the "data page rewrite" phase won't be paced. It seems
surprising the first phase would not be throttled by default, while the
checkpoints are ...

So I ended up setting the 'fast=true' default.

This is not a very complicated patch, and it's been part of the old
patch series for quite a while. So unless there's some objections I'll
get this committed in a couple days.

regards

--
Tomas Vondra