Re: Changing the state of data checksums in a running cluster

The attached rebase contains lots more polish, mostly renaming variable names
for clarity, tidying up comments and documentation and some smaller bits of
cleanup like moving more code out of xlog.c.

This version runs all the tests in a normal test-run, with a few of them pared
down with larger runs gated by PG_TEST_EXTRA. I thinkt the tests are still too
expensive in the event of getting committed, but it's helpful to have them
during dev and test. Executing pgbench sometimes fails in CI but I've been
unable to reproduce that so not entirely sure what is going on there.

Heikki, Andres and Tomas; as you have been reviewing this patchset, what do you
feel is left for considering this for commit? (Apart from figuring out the CI
test thing mentioned above which I think is a buildsystem issue.) I think 0001
could be considered independently of 0002 and is cleanup in it's own right.

--
Daniel Gustafsson