Confine vacuum skip logic to lazy_scan_skip

Hi,

I've written a patch set for vacuum to use the streaming read interface
proposed in [1]. Making lazy_scan_heap() async-friendly required a bit
of refactoring of lazy_scan_heap() and lazy_scan_skip(). I needed to
confine all of the skipping logic -- previously spread across
lazy_scan_heap() and lazy_scan_skip() -- to lazy_scan_skip(). All of the
patches doing this and other preparation for vacuum to use the streaming
read API can be applied on top of master. The attached patch set does
this.

There are a few comments that still need to be updated. I also noticed I
needed to reorder and combine a couple of the commits. I wanted to
register this for the january commitfest, so I didn't quite have time
for the finishing touches.

- Melanie

[1] https://www.postgresql.org/message-id/flat/CA%2BhUKGJkOiOCa%2Bmag4BF%2BzHo7qo%3Do9CFheB8%3Dg6uT5TUm2gkvA%40mail.gmail.com