WAL format changes break the suppression of do-nothing checkpoints.

commit 2c03216d831160bedd72d45f7 has invalidated the part of the docs
saying "If no WAL has been written since the previous checkpoint, new
checkpoints will be skipped even if checkpoint_timeout has passed",
presumably by accident.

It seems that this part is no longer true when it should be true:

if (curInsert == ControlFile->checkPoint +
MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint))

MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint) is now 96, but the amount by
which curInsert gets advanced is still 104, like it was before the commit.

Cheers,

Jeff