Page at a time index scan

Here's a patch that implements page at a time index scans discussed at
pgsql-hackers earlier. See proposal 1 at:
http://archives.postgresql.org/pgsql-hackers/2006-03/msg01237.php

It passes regression tests, and there's no known bugs. There's
some minor issues I'd like to point out, though:

1. An index scan now needs to allocate enough memory to hold potentially a
whole page worth of items. And if you use markpos/restrpos, twice that
much. I don't know if that's an issue, but I thought I'd bring that up.

2. Vacuum is now done in one phase, scanning the index in physical order.
That significantly speeds up index vacuums of large indexes that don't fit
into memory. However, btbulkdelete doesn't know if the vacuum is a full or
lazy one. The patch just assumes it's a lazy vacuum, but the API really
needs to be changed to pass that information earlier than at vacuum_cleanup.

3. Before the patch, a scan would keep the current page pinned to keep
vacuum from deleting the current item. The patch doesn't change that
behaviour, but it now seems to me that even a pin is no longer needed.

The patch needs testing and review, to ensure it doesn't brake anything,
and to see the effect on performance. It doesn't change disk layout or
catalogs, so you can run it using the same data directory as with the
unpatched version.

- Heikki