Re: Enabling Checksums

New version of checksums patch.

Changes:
  * rebased
  * removed two duplicate lines; apparently the result of a bad merge
  * Added heap page to WAL chain when logging an XLOG_HEAP2_VISIBLE to
avoid torn page issues updating PD_ALL_VISIBLE. This is the most
significant change.
  * minor comment cleanup

No open issues that I'm aware of with the patch itself.

Greg appears to have made some progress on the automated corruption
tester.

Note to reviewers: I also have a patch out to remove PD_ALL_VISIBLE
entirely. The code around PD_ALL_VISIBLE is quite tricky (with or
without this patch), so if the PD_ALL_VISIBLE patch is committed first
then it will make reviewing this patch easier. Regardless, the second
patch to be committed will need to be rebased on top of the first.

Regards,
	Jeff Davis