Patch for seq scan & scan resistant bufmgr

Hi,

I am submitting a patch for review that:

1. for small relation (smaller than 60% of bufferpool), use the
current logic
2. for big relation:
    - use a ring buffer in heap scan
    - pin first 12 pages when scan starts
    - on consumption of every 4-page, read and pin the next 4-page
    - invalidate used pages of in the scan so they do not force out
other useful pages

4 files changed:
bufmgr.c, bufmgr.h, heapam.c, relscan.h

If there are interests, I can submit another scan patch that returns
N tuples at a time, instead of current one-at-a-time interface. This
improves code locality and further improve performance by another
10-20%.

For TPCH 1G tables, we are seeing more than 20% improvement in scans
on the same hardware.

------------------------------------------------------------------------
-
----- PATCHED VERSION
------------------------------------------------------------------------
-
gptest=# select count(*) from lineitem;
   count
---------
6001215
(1 row)

Time: 2117.025 ms

------------------------------------------------------------------------
-
----- ORIGINAL CVS HEAD VERSION
------------------------------------------------------------------------
-
gptest=# select count(*) from lineitem;
   count
---------
6001215
(1 row)

Time: 2722.441 ms


Suggestions for improvement are welcome.

Regards,
-cktan
Greenplum, Inc.