Improve truncation of pg_serial/, removing "apparent wraparound" LOGs

Improve truncation of pg_serial/, removing "apparent wraparound" LOGs

It is possible that the tail XID of pg_serial/ gets ahead of its head
XID, which would cause the truncation of pg_serial/ done during
checkpoints to show up as a "wraparound" LOG in SimpleLruTruncate(),
which is confusing.  This also wastes a bit of disk space until the head
page is reclaimed again.

CheckPointPredicate() is changed so as the cutoff page for the
truncation is switched to the head page if the tail XID has advanced
beyond the head XID, rather than the tail page.  This prevents the
confusing LOG message about a wraparound while allowing some truncation
to be done to cut in disk space.

This could be considered as a bug fix, but the original behavior is
harmless as well, resulting only in disk space temporarily wasted, so
no backpatch is done.

Author: Sami Imseih
Reviewed-by: Heikki Linnakangas, Michael Paquier
Discussion: https://postgr.es/m/755E19CA-D02C-4A4C-80D3-74F775410C48@amazon.com