Clear up issue with FSM and oldest bpto.xact.

Clear up issue with FSM and oldest bpto.xact.

On further reflection, code comments added by commit b0229f26 slightly
misrepresented how we determine the oldest bpto.xact for the index.
btvacuumpage() does not treat the bpto.xact of a page that it put in the
FSM as a candidate to be the oldest deleted page (the delete-marked page
that has the oldest bpto.xact XID among all pages encountered).

The definition of a deleted page for the purposes of the bpto.xact
calculation is different from the definition used by the bulk delete
statistics.  The bulk delete statistics don't distinguish between pages
that were deleted by the current VACUUM, pages deleted by a previous
VACUUM operation but not yet recyclable/reusable, and pages that are
reusable (though reusable pages are counted separately).

Backpatch: 11-, just like commit b0229f26.