pgsql: Reduce impact of btree page reuse on Hot Standby by fixing off-b

Reduce impact of btree page reuse on Hot Standby by fixing off-by-1 error.
WAL records of type XLOG_BTREE_REUSE_PAGE were generated using a
latestRemovedXid one higher than actually needed because xid used was
page opaque->btpo.xact rather than an actually removed xid.
Noticed on an otherwise quiet system by Noah Misch.

Noah Misch and Simon Riggs

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ca7e04b0b9943d5c33d2e993ff685d45b5bddbbd

Modified Files
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src/backend/access/nbtree/nbtpage.c |   12 +++++++++++-
1 files changed, 11 insertions(+), 1 deletions(-)