pgsql: Prevent index-only scans from returning wrong answers under Hot

Prevent index-only scans from returning wrong answers under Hot Standby.

The alternative of disallowing index-only scans in HS operation was
discussed, but the consensus was that it was better to treat marking
a page all-visible as a recovery conflict for snapshots that could still
fail to see XIDs on that page. We may in the future try to soften this,
so that we simply force index scans to do heap fetches in cases where
this may be an issue, rather than throwing a hard conflict.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/3424bff90f40532527b9cf4f2ad9eaff750682f7

Modified Files
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src/backend/access/heap/heapam.c | 18 ++++++++++++++++--
src/backend/access/heap/visibilitymap.c | 9 ++++++---
src/backend/commands/vacuumlazy.c | 11 +++++++++--
src/include/access/heapam.h | 2 +-
src/include/access/htup.h | 3 ++-
src/include/access/visibilitymap.h | 2 +-
src/include/access/xlog_internal.h | 2 +-
7 files changed, 36 insertions(+), 11 deletions(-)