pgsql: Avoid having two copies of the HOT-chain search logic.

Avoid having two copies of the HOT-chain search logic.

It's been like this since HOT was originally introduced, but the logic
is complex enough that this is a recipe for bugs, as we've already
found out with SSI. So refactor heap_hot_search_buffer() so that it
can satisfy the needs of index_getnext(), and make index_getnext() use
that rather than duplicating the logic.

This change was originally proposed by Heikki Linnakangas as part of a
larger refactoring oriented towards allowing index-only scans. I
extracted and adjusted this part, since it seems to have independent
merit. Review by Jeff Davis.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/4da99ea4231e3d8bbf28b666748c1028e7b7d665

Modified Files
--------------
src/backend/access/heap/heapam.c | 72 ++++++++-----
src/backend/access/index/genam.c | 4 +-
src/backend/access/index/indexam.c | 174 ++++-------------------------
src/backend/executor/nodeBitmapHeapscan.c | 4 +-
src/include/access/heapam.h | 3 +-
src/include/access/relscan.h | 4 +-
6 files changed, 74 insertions(+), 187 deletions(-)