pgsql: Fix race condition between hot standby and restoring a full-page

Fix race condition between hot standby and restoring a full-page image.

There was a window in RestoreBackupBlock where a page would be zeroed out,
but not yet locked. If a backend pinned and locked the page in that window,
it saw the zeroed page instead of the old page or new page contents, which
could lead to missing rows in a result set, or errors.

To fix, replace RBM_ZERO with RBM_ZERO_AND_LOCK, which atomically pins,
zeroes, and locks the page, if it's not in the buffer cache already.

In stable branches, the old RBM_ZERO constant is renamed to RBM_DO_NOT_USE,
to avoid breaking any 3rd party extensions that might use RBM_ZERO. More
importantly, this avoids renumbering the other enum values, which would
cause even bigger confusion in extensions that use ReadBufferExtended, but
haven't been recompiled.

Backpatch to all supported versions; this has been racy since hot standby
was introduced.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/681dbe7d4599b4b13f9039ac961be14988120c1d

Modified Files
--------------
src/backend/access/hash/hashpage.c | 13 +++++-----
src/backend/access/heap/heapam.c | 3 +--
src/backend/access/transam/xlog.c | 6 +----
src/backend/access/transam/xlogutils.c | 17 +++++++++----
src/backend/storage/buffer/bufmgr.c | 41 ++++++++++++++++++++++++++++----
src/include/storage/bufmgr.h | 11 ++++++---
6 files changed, 66 insertions(+), 25 deletions(-)