pgsql: Fix off-by-one error in calculating subtrans/multixact truncatio

Fix off-by-one error in calculating subtrans/multixact truncation point.

If there were no subtransactions (or multixacts) active, we would calculate
the oldestxid == next xid. That's correct, but if next XID happens to be
on the next pg_subtrans (pg_multixact) page, the page does not exist yet,
and SimpleLruTruncate will produce an "apparent wraparound" warning. The
warning is harmless in this case, but looks very alarming to users.

Backpatch to all supported versions. Patch and analysis by Thomas Munro.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/766dcfb16ca385274d510eaed01724bb3836efdd

Modified Files
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src/backend/access/transam/multixact.c | 13 ++++++++++---
src/backend/access/transam/subtrans.c | 7 ++++++-
2 files changed, 16 insertions(+), 4 deletions(-)