pgsql: Don't clear btpo_cycleid during _bt_vacuum_one_page.

Don't clear btpo_cycleid during _bt_vacuum_one_page.

When "vacuuming" a single btree page by removing LP_DEAD tuples, we are not
actually within a vacuum operation, but rather in an ordinary insertion
process that could well be running concurrently with a vacuum.  So clearing
the cycleid is incorrect, and could cause the concurrent vacuum to miss
removing tuples that it needs to remove.  This is a longstanding bug
introduced by commit e6284649b9e30372b3990107a082bc7520325676 of
2006-07-25.  I believe it explains Maxim Boguk's recent report of index
corruption, and probably some other previously unexplained reports.

In 9.0 and up this is a one-line fix; before that we need to introduce a
flag to tell _bt_delitems what to do.

Branch
------
REL8_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/2c293f25494f2d465bcff49d7aab6ec06a40e436

Modified Files
--------------
src/backend/access/nbtree/nbtinsert.c |    2 +-
src/backend/access/nbtree/nbtpage.c   |   10 ++++++----
src/backend/access/nbtree/nbtree.c    |    2 +-
src/include/access/nbtree.h           |    2 +-
4 files changed, 9 insertions(+), 7 deletions(-)