Single pass vacuum - take 2

Hi All,

Here is a revised patch based on our earlier discussion. I implemented
Robert's idea of tracking the vacuum generation number in the line
pointer itself. For LP_DEAD line pointers, the lp_off/lp_len is unused
(and always set to 0 for heap tuples). We use those 30 bits to store
the generation number of the vacuum which would have potentially
removed the corresponding index pointers, if the vacuum finished
successfully. The pg_class information is used to know the status of
the vacuum, whether it failed or succeeded. 30-bit numbers are large
enough that we can ignore any wrap-around related issues. With this
change, we don't need any additional header or special space in the
page which was one of the main objection to the previous version.

Other than this major change, I have added code commentary at relevant
places and also fixed the item.h comments to reflect the change. I
think the patch is ready for a serious review now.

Thanks,
Pavan

--
Pavan Deolasee
EnterpriseDB     http://www.enterprisedb.com