Online checksums patch - once again

OK, let's try this again :)

This is work mainly based in the first version of the online checksums
patch, but based on top of Andres WIP patchset for global barriers (
https://www.postgresql.org/message-id/20181030051643.elbxjww5jjgnjaxg%40alap3.anarazel.de
)

Andres patch has been enhanced with wait events per
https://www.postgresql.org/message-id/CABUevEwy4LUFqePC5YzanwtzyDDpYvgrj6R5WNznwrO5ouVg1w%40mail.gmail.com
.

I'm including both those as attachments here to hopefully trick the cfbot
into being able to build things :)

Other than that, we believe that the list of objections from the first one
should be covered by now, but it's been quite some time and many emails, so
it's possible we missed some. So if you find them, point them out!

Documentation needs another go-over in particular base don changes since,
but the basic principles of how it works should not have changed.

//Magnus & Daniel