Rename delayChkpt to delayChkptFlags.

Rename delayChkpt to delayChkptFlags.

Before commit 412ad7a55639516f284cd0ef9757d6ae5c7abd43, delayChkpt
was a Boolean. Now it's an integer. Extensions using it need to be
appropriately updated, so let's rename the field to make sure that
a hard compilation failure occurs.

Replacing delayChkpt with delayChkptFlags made a few comments extend
past 80 characters, so I reflowed them and changed some wording very
slightly.

The back-branches will need a different change to restore compatibility
with existing minor releases; this is just for master.

Per suggestion from Tom Lane.

Discussion: http://postgr.es/m/a7880f4d-1d74-582a-ada7-dad168d046d1@enterprisedb.com