pgsql: Change currtid functions to use an MVCC snapshot, not SnapshotNo

Change currtid functions to use an MVCC snapshot, not SnapshotNow.

This has a slight performance cost, but the only known consumers
of these functions, known at the SQL level as currtid and currtid2,
is pgsql-odbc; whose usage, we hope, is not sufficiently intensive
to make this a problem.

Per discussion.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ed93feb80891b131e9676e962256cc2b18aa5e78

Modified Files
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src/backend/utils/adt/tid.c |   12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)