Use GlobalVisState in vacuum to determine page level visibility

Use GlobalVisState in vacuum to determine page level visibility

During vacuum's first and third phases, we examine tuples' visibility to
determine if we can set the page all-visible in the visibility map.

Previously, this check compared tuple xmins against a single XID chosen
at the start of vacuum (OldestXmin). We now use GlobalVisState, which
enables future work to set the VM during on-access pruning, since
ordinary queries have access to GlobalVisState but not OldestXmin.

This also benefits vacuum: in some cases, GlobalVisState may advance
during a vacuum, allowing more pages to become considered all-visible.
And, in the future, we could easily add a heuristic to update
GlobalVisState more frequently during vacuums of large tables.

OldestXmin is still used for freezing and as a backstop to ensure we
don't freeze a dead tuple that wasn't yet prunable according to
GlobalVisState in the rare occurrences where GlobalVisState moves
backwards.

Because comparing a transaction ID against GlobalVisState is more
expensive than comparing against a single XID, we defer this check until
after scanning all tuples on the page. Therefore, we perform the
GlobalVisState check only once per page. This is safe because
visibility_cutoff_xid records the newest live xmin on the page; if it is
globally visible, then the entire page is all-visible.

Using GlobalVisState means on-access pruning can also maintain
visibility_cutoff_xid, which is required to set the visibility map
on-access in the future.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/flat/bqc4kh5midfn44gnjiqez3bjqv4zogydguvdn446riw45jcf3y%404ez66il7ebvk#c755ef151507aba58471ffaca607e493